Is the RS tour of Turkey as strenuous as the graph depicts ?

nivende, it’s a great country and a wonderful tour, which I took in 2018. Like on most RS Tours there is a lot of walking, sometimes including steps, but not long, long climbs as I remember. No bell towers or Duomo domes to climb. Of course each traveler controls their own ambitions and pace to some extent, in exploring the various ruins, and in your free time. Walk a lot, take a hike, or find a coffee and observe. There were a couple experiences that I, and some others, abbreviated or skipped (underground city, where I understood you needed to explore bent over). Once you’ve settled into the tour, I would take the guide aside and explain your concerns or limitations, and he/she can advise you of challenges as you go through the tour. The tour spends little time in Istanbul, a huge city with many diverse areas and wonderful sites, so I would definitely advise you to spend some extra days in Istanbul before and/or after the tour. But go and enjoy!

We did this tour in May/June 2018 and of the three RS tours we have done to date this is both a very active and fabulous tour.
I do not know what a typical 70 year old is by your definition, but I was 72 and my partner was 73, she is in terrific shape and I am in very good shape. Both of us exercise daily.
There are few things to consider, the local temperatures when you go, the actual pace of the tour and the amount of non-tour activities you choose.
Some of the days were quite warm such as the day we were at Ephesus and we are not great fans of really hot weather.
The pace of the tour is what we have found to be typical RS tour, easy to strenuous, but we had no problems with any day of the tour. The tour was more strenuous than our Eastern Europe or Portugal tour.
We also like to be quite active during our free time. That said, there was not a great deal of free time as our excellent guide offered a number of extra activities that most of us participated in. The bottom line here is that you might choose to use your free time in ways that might or might not add the the overall activity level.
I do not recall an unusual amount of stairs, but your wife's knee issues might be a concern. Perhaps a call to the RS office might be worthwhile.
All of this said, we still find ourselves talking about how much we enjoyed this tour.

I don’t remember a lot of steps on this tour. And yes, a normal 70 year old will be fine on this tour. Surfaces were uneven at times so watch where you step. We went the last of September and our issue was the heat! It was not unbearable, but it was warm. I thought our day in Amsterdam (another RS tour) a lot more strenuous (walking) than the Turkey tour.
The visit at Ephesus and Cappadocia does have a lot of walking on uneven surfaces. You can sit in these areas and visit at you own pace, as you are set free at some point at these sites to explore as much or as little as you want.
If you don’t have a bad back or neck issue, don’t skip the underground city. You don’t have to bend down all the time or for long. My husband is tall and had a severe fall (one story) resulting in life long pain and stiffness. He was able to do it.

I found the Turkey tour similar in activity to the other RS tours I have taken (Scandinavia and Italy). However, stairs cannot be avoided. For example: the hotel in Istanbul requires that you go down approximately 15 stairs to get to the lobby. There is a very nice but small elevator to get to the rooms, but it doesn't go all of the way up to the breakfast floor, so another flight of stairs there. The hotel in Cappodocia also has a steep flight of stairs up to the breakfast room.
I am relatively young (54) but I have no meniscus in my left knee and the right knee isn't much better.
I did fine on this tour with the help of a hunger hiker walking stick.
I did skip a few things. I did not climb to the top of the ancient theaters. I did not climb to see the frescoes in the highest cave churches in Goreme. I did not go on the optional hike in Cappodocia. I did not go very far underground at Kaymakli.
I had a great time on this tour and I learned so very much.

I recommend travel with a pair of very light collapsible hiking poles that spare the joints and assure balance. They really were a lifesaver in Greece.

I developed a knee problem on Day 2 of the tour. Uneven surfaces, up/down terrain and stairs were all somewhat painful, stairs much more than "somewhat". There was an optional hike after the optional balloon ride. I went on the balloon, sat out the hike. There were one or two other activities that were for 1-2 hours that I also skipped, ended up playing a lot of backgammon with the bus driver which was just as good. The only place I really had a problem was Ephesus, which meant that I didn't go to as many bits as the others did. I don't remember having to go up more than one flight of stairs at any of the hotels.

Turkey is one of 3 RS tours I've taken and the least difficult. Greece was much more up and down (managed fine) and the Alpine tour which was "choose your own level of activity."

I can't report on a Rick Steves tour specifically but just came back from a trip that included a full day in Ephesus: the House of the Virgin Mary (questionable claims to authenticity) , the wonderful ancient city of Ephesus which included the Terrace Steps, St. John's Basilica, and the Temple of Artemis. The only area that might be challenging is the Terrace Steps because it has lots of steps and the staircase down is pretty challenging without railing!