Some of the features you mention are OS-level features and every app therefore offers them out of the box (like zoom in) without developer support.
As for them "choosing" not to implement certain accessibility features on launch, every launch involves choices. I don't know how you can tell me with a straight face that 40 million people with vision problems are more important than 400 million people in EU in term of prioritization. But you did, anyway.
As a developer, there's no such thing as "basic accessibility features" that just come with "the tools." You need to figure out a good UI, implement it, test it, set limits to preserve function but avoid abuse and so on.
Regarding ALT, I wonder, what stops accessibility mindful people to just describe the image in their post, for example? But I guess if you don't have it conveniently as ALT text on the image itself, then screw blind people, and skip directly to outrage.
People who need accessibility are important to consider, but they're not more important than other people who need to be considered. They're definitely not ten times more important than other people. I believe we're leaning here into this toxic outrage culture society has adopted, which promotes overreacting to issues of disability, race, sexual orientation. It's becoming increasingly unsustainable. I suppose it's natural for Mashable to be scrambling to cause some outrage for the clicks, but it's still on us to know better.
We're all people and we're all the same. No one's issues are more important than another person. Blind people deserve the same as everyone, but it just so happens in reality support for them is additional work, therefore that work will have to be done additionally. I'm in Europe and I have no access to the app at all, but I'm not complaining. GDPR requires additional work, and it'll have to be done additionally. I'll wait for Meta to get to me. Blind people can also survive without posting on Threads for a few months. Threads is not an essential service to absolutely anyone, especially 2 days out of the gate.