Threads was launched before it was finished to capitalise on one of Musk’s more brain dead decisions - and the Threads website was initially almost unusable (it’s still missing a lot of things - such as federation with Mastodon).
The requirement to use a phone app, if it still exists, wasn’t nefarious, it was just the best they could do without delaying the product launch.
As for “the account they already apparently created”… AFAIK threads runs on Instagram’s infrastructure. So if you have an Instagram account then, yeah, “signing up to threads” is basically just enabling threads on your existing account. If you don’t have an Instagram account I’m pretty sure they create one when you sign up for Threads (but disable Instagram on your account unless you “sign up” over there).