As said above its whatever is more comfortable/natural to you. I ride goofy (right foot forward), used to skateboard right foot forward, right foot forward when pedals level on bike, etc etc. I’m right footed in that I would kick something with my right foot, but I’m rubbish at football, so not sure if I really am with co-ordination.
Basically the ice slidy thing is a way of finding out which you’re more comfortable with. Alternatively spin round lots, get really dizzy, then get somebody to shove you from behind, you will instinctively put one foot forward. This foot is the one you should have forward. Especially if it felt weird the other way round.
After a couple of weeks I started trying to learn switch (left foot forward just by going down the hill ‘backwards’) as its useful occasionally. But start off with what you feel comfortable with. I’m no expert but your instructor sounds like a tool.