If you run IG for your business - as I (have to) do - being able to upload properly edited images directly from the Mac and writing text on a proper keyboard is really advantageous - no need to airdrop images to the phone, no need to clumsily type longer text on the tiny touchscreen. Since, as you mentioned, IG only has an iPhone app which does not autorotate on an iPad, even the iPad Pro with Magic Keyboard is not real option.
This is a corner case I agree, but I guess many of us, particularly those 'influencers', would rejoice about this new possibility.
Magnus