Unknown Thieves

I was looking at the guidelines given in the Expert Rulebook for "levels beyond those listed," which offers some advices to referees looking to expand the rules to cover levels 15 and up. In the section describing thieves, it says:

It will be noted that thieves have high chances of success in their special abilities when conditions are favorable for that action. Thieves will therefore gain new abilities requiring greater skill and danger. These will include the ability to climb overhangs, upside down, ventriloquism, powers of distraction, and the ability to mimic voices.

Somehow, I never noticed that passage before or, if I did, I don't remember reading it. I'm not sure what I think about their possible inclusion or how their inclusion might have affected my estimation of the thief's place in D&D. Still, this is the first time I've seen a suggestion to expand the thief's skills in old school D&D beyond the thief-acrobat in Unearthed Arcana, but I do find it very intriguing, if only because it reveals avenues for further developing the class that are consonant with its origins rather than pushing it toward the ninja death commando of later editions.