I don't know.
Whether or not you feel that they are official by that part, the difference is communication. Communication is always a key theme in Cloud-Tifa, even moreso than any other Final Fantasy couple.
Cloud's inability to communicate with Tifa, minus that one time at the water tower, stopped them from really being in a relationship when they were kids. It stopped him from telling Tifa who he was during the flashback. It stopped Tifa from telling Cloud how she felt, throughout the game and highlighted in the date. And it stopped her from telling Cloud about his true past. Their inability to communicate harmed them in CoT and caused Tifa to forcibly tear down the walls in AC/C with her confronting him.
Communication is their number one problem. And in the highwind in HA, it's something they overcome. Granted, it's a large part of it solved by the lifestream, but the difference between high affection and low affection is how they further progress. If Cloud and Tifa still can't show their feelings are are uncomfortable with it, then even if the results are the same, that shows something different than if they "showed that they matched their feelings without words."
IMO, I consider high affection more canon than low affection the same way I view Aerith date more canon than Tifa/Yuffie/Barret. Given the nature of what happens before (or after), it just makes sense.
...
Also CoT and AC/C was, like, seven years later so all my carefully hypothesized guesses make more sense in that context