Just my two cents on the topic, but I'm guessing that Instagram was never really mean't to be used on desktop – it was developed mobile-first and just ported to desktop when there was a bigger demand for it.
I would think that since most of their content (photos, stories,..) are optimized for mobile screen size only.
So if their infrastructure doesn't consider much for responsiveness between multiple screen size, I doubt the backend for the web version matter much for them.