I'd be surprised if they even care about that, but who knows, it's a possibility
It's a legal requirement in some parts of the world. Oh, and it means a bigger potential userbase for them. Plus it's just generally an all-round considerate thing to do.
Why would you be surprised? They put in effort for their iOS apps to work properly (and the alt-text tags are used for Instagram in "blind mode" for example).
If you're a company that wants personal data from everyone, you want personal data from everyone.
You'd be surprised if the largest content delivery platform in the world cares about being accessible to the broadest range of users?