Sometime before she began making so many pies for Rainbow Dash, Pinkie invented/started using edible pie tins.
It would explain why not once in those years did she question what happened to the tins for the pies she was making, as well as why Tank was unharmed by swallowing the tin whole. Rainbow has, or at least used to have, some kind of sensitivity(allergy, intolerance, etc.) to gluten.
If there were just certain flavors of pies she didn't like, that'd be one thing, but for her to hate every kind of pie, regardless of the flavor or who made it, means it has to have something to do with the crust. This would also explain why, even though it was obvious that Pinkie Pie was figuring out the ruse, Rainbow didn't just put on a brave face and eat one—it's one thing to have to just wash a bad taste out of your mouth, but it's another to have to deal with being seriously ill later(and it speaks volumes that she later decided that that'd be Worth It to make up for how much she hurt Pinkie).Even if it was an allergy she outgrew, she could still be suffering from a case of taste aversion
; one too many bad reactions back when it was still a problem left it so that anything that used to make her sick still disgusts her to eat, even if now it would be harmless.
