> I never saw Instagram as appealing to photography hobbyists. Instead, I saw it as deliberately nerfing things where hobbyists have advantages (image quality, choice of aspect ratios, posting from desktop PCs), likely to increase participation by making it less intimidating to share snapshots taken on phone cameras.

I agree with this 100%, on top of what you said remember that Instagram launched in 2010 as an iOS exclusive during a time where Apple was not particularly focused on camera quality, ignoring Android where there were numerous devices with substantially better cameras. IIRC someone was even selling one with an optical system in the ballpark of a low-end mirrorless. They also limited image resolution to 640 pixels square until 2015.