So how is this different from people sitting in front of a TV and watching endless samey series?

Only that it's portable.

If we didn't have "social media" we'd be all watching samey tv series on our phones.

It absolutely makes a difference because tv shows are usually 20 mins at least, which means watching 3 minutes in the supermarket line is actually a bad experience, so it requires more deliberation.

I’d also argue that the average TV show is more edifying than the average social media post but that’s another topic.