So, companies will create Threads accounts and use them to post ads as regular Threads posts, then? Because that's what the block button is for.

There's also nothing that stops any company from doing exactly this in any instance that currently exists, including spinning up their own instance.

If we're talking ads that Threads will be inserting into the feeds on their site, what does it matter? Either they are a user making a regular post treated like any other user or they are something that Threads can only push on their own site.