I can sort of see where they are going with the argument.
Playing devil's advocate for a bit: Tifa's section treats the affectionate highwind scene as canon in its wording. But it does the same thing with the date sequence.
Looking at Aerith's section, it also treats the date sequence as canon, no mention of variability. They both can't be canon, unless Cloud's also a really smooth time traveling player, which is a wonderful story for a sequel or something, but they still are written like they would be in their section- and there's no saying which is the "right" date.
It's put in there because it's significant to the character, not because its canon. Therefore there's no stretch of the imagination that the high affection scene has the same treatment- it's mentioned because its memorable but the variability still exists.
I guess my counterargument to that is the fact the date sequences reveal something true about the characters, and that just because they don't have that date doesn't mean that Tifa can't spit her feelings out or that Aerith doesn't know Cloud's messed up in the head- so all those romantic feelings are still there. And the highwind scene specifically mentions the romantic feelings they have towards each other.
That and the date scene =/= highwind scene in terms of importance. The lifestream scene is a natural segue to the highwind scene, which is a natural segue to the "I have you this time" and then to the starting a life together thing.
Still can't blame them from raising the point though.
Now where do I put the money? 
