And what Rick Steves refers to as "guides" are technically tour directors or tour managers.
Here is how the tour industry sees it:
Tour Guide: Tour guides offer specific narration in a destination and are often called step-on guides. When a tour group arrives at their planned destination is an advantage in having a local guide join the group for the day or even a few hours. Living and working in the destination provides for a more intimate connection and these guides can add a personal glimpse of the community into their commentary.
Tour Director: Tour Directors are responsible for the on the road logistics, confirmations, planning, unexpected delays, damage control, and group dynamics. They travel with the group for the entire tour and can be away from home for weeks or months depending on the tour. During a tour they are available 24/7.
So, Rick calls his "tour directors" guides so he can compare them to other company's guides and not tour directors who are usually professionally trained and licensed.
You can call these things anything you want. If you call a coach a bus, you can do it but you are technically wrong. If you call a tour director a guide, you can do it but you would be technically wrong.
But more importantly, why do I bother?