I've done THREE Ironman swims and I'm STILL scared of the Masters classes.
Around here they are for what I would call the grown up fishies.
Fast adult swimmers who want to get faster.

When I was first getting into the sport I signed up for an adult swim lesson session at the Y. I told the teacher on our first day why I was there, and she worked with me a bit different than the others. I was a lot like you. I basically knew how to swim, but was never a "real" swimmer. It was a great experience.
I'm still slow as molasses in january in the water, but I make the cut offs.
Personally I'm ready to progress on to the local tri coaches who do a total immersion private program. It's spendy tho, and being a biz owner I'm a bit short of funds these days.
As for the masters... The instructor sounds like a turd. I wouldn't go back. In fact I don't think I would have gone back for a second session.
BTW- are there other triathletes at that class? I'm wondering if he has a thing against triathletes. Some ( a lot actually) single sport coaches seem to have a chip on their shoulders.
I have heard the phrase "jack of all trades, master of NONE" far too many times...