The concept of a handle goes beyond a username. If someone can construct a profile that looks like someone's profile on another site and contains approximately the words in the username, like _username or imusername instead of username, they might be able to impersonate it. In that case it would be good to have an active profile on that platform to counteract it.

Not the case on Twitter. It becomes instantly available to anyone who wants it.

Well that seems... dumb.

Yup, about par for the course with Twitter these days sadly.

Well, it's a Twitter product decision, so, yes, 'dumb' is about what one would expect.