Yep it's true. The prior route described in the older RS books (from the Colosseum straight up through the Arch of Titus) is gated off and you can't get through.
There are two ways to get into the Forum. I'm going off of memory and they're kind of confusing, so I'll try to explain it as best as I can.
The first entrance, and the way we did it, is from Palentine Hill. From the Colosseum entrance, as you are facing the Forum, you walk to the left for several minutes (I think it was something like 500-700 meters?) until you reach the Palentine Hill entrance. Your Colosseum ticket is good for Palentine + Forum, as is the Roma Pass if you choose to go that route. Passing through the entrance of the Palentine Hill, there is a long stairway to the right which leads to the Forum. I really enjoyed touring the Palentine Hill, which has some spectacular views, so depending on how much time you have, you might consider doing that as well.
The staircase puts you on the inside of the gated area by the Arch of Titus, and from there you can walk down into the Forum "proper". I don't recall exactly where the RS guided tour starts you off from. Are you using the audio podcast? While very informative and I do recommend it, the Forum podcast tends to double back on itself a lot and you end up running back and forth a bit.
The other entrance, which we did not use, I am not 100% certain of, but I believe it is along the Via dei Fori Imperiali, which is the major road that runs up with the Roman Forum on the left and Trajan's Forum on the right. The street was under a TON of construction last November. I believe the entrance on this side is nearer the end of RS's tour, by the Arch of Septimus Severus, and about halfway up the stairs that lead to the Campidoglio. I would say it is probably further from the Colosseum to this entrance than to the Palentine Hill and will probably take you 10 minutes or so to walk.