If you buy it in advance, it's not a souvenir. It's just some stuff you bought, nothing else. The concept of a souvenir isn't really complicated.
That comment was not nice, and doesn't take into account that some families are on budgets, some want control over what is brought home, and some [like me] just don't want their kids to associate WDW with a "gimme, gimme, gimme" shopping spree at the exit of every ride.
In truth, this is what a souvenir is: a memento, keepsake, or token of remembrance.
I brought along a plush for each of my kids on our first trip. "Mickey" left one for each of them on their beds while we were at the park on our first day. It is a treasured souvenir, because for them it is the memory of coming back to the room and seeing a gift just for them.
We told them at the beginning of that trip that we would go to World of Disney on our last night and they could each pick out one item. Guess what? They picked out crap toys that were soon broken/lost.... but each of them still has the plush I bought ahead of time, and have taken them on every WDW trip since.
One our last trip, our oldest couldn't decide on his one item... nothing appealed to him. He asked if he could look online when he got home instead. He found something he wanted online, ordered it, and it absolutely is - for him - a souvenir of that trip. Doesn't matter one little bit that I didn't carry it home in a suitcase.
OP: Whatever works for your family, that's what you should do!