tbh I wouldn't see the instagrram accounts as canon
yeah ik, I also don't think any of astruc's tweets are canon in the exact same way for the exact same reason. His tweets are the only place we get explicit confirmation that Adrien 100% doesn't see Mari as romantic in the slightest; the show itself leaves room for doubt and can be interpreted in many different ways.
Both the IG accounts and his Twitter exist to give us extra content to support the canon show even if neither are strictly canon.
I'm half asleep rn so I'll probably come back to this in a few hours, but my point is: even if the ig accounts aren't canon, they're at least canon compliant material provided by the network that determines what's canon, so it's not that much of a stretch for the audience to see the 'just a friend' line used for both Mari and Kagami -> recall that Adrien does in fact have a canon crush on Kagami -> interpret this as a pattern of behavior that Adrien has about his crushes -> assume that he might have the slightest crush on Mari.
