Are you willing to turn down the job based on the fact you may not be supervising people? Is that the only reason you were considering leaving your current position?
I mean, if it's a deal breaker for you, it's a deal breaker, but I certainly wouldn't base any decisions on "inside" information. I mentioned to a coworker that there was a job opening at a medical facility very close to me and she immediately started in about how I shouldn't even consider it because it's a very stressful and messed up place, she's heard really bad things, etc. She depends on me a lot and didn't want me to leave, she didn't know, and still doesn't know that I have already worked there. The position ended up being filled from within and absorbed by current staff members, but my point is that sometimes people have their own motives for saying things.
Without exaggeration, every single job I've been hired for has ended up having nothing to do with the job description I agreed to and not in a bad way. I had been laid off from a job and accepted a part time receptionist position while I looked for something else. My first day, I was promoted to a managerial position. Who could have seen that coming? A guy had gotten married and was relocating, they saw my skills and bang! Done deal. Full time, full benefits. Score! Every job I've ever had, I end up doing MORE than I was hired for.
Personally, I wouldn't count this job out. You haven't even been trained yet, how are you just supposed to start out supervising people when you don't even know their procedures yet? Not only that, you weren't informed of this by the actual HR department or hiring manager(s). I've worked with lots of people who think they know everything that goes on and they don't. I work in HR and I would NEVER discuss something with someone I know personally. If I were to get wind that someone was being hired under a certain pretence and it wouldn't be productive to hire someone who wouldn't be happy, I would go a step above and let them handle it. I wouldn't call the person and say, "Hey...guess what I heard....".
I've worked with people I know who've gotten fired. It's not an easy situation, but I can't discuss that with them on personal time.
TOTALLY unprofessional.
If you ONLY want this new position IF you will be supervising people, call whoever interviewed you and make that clear. However, you could be shooting yourself in the foot and talk yourself out of a good opportunity. If you'd rather stay where you are under those circumstances, nothing lost and nothing gained.
Just my opinion.