"But I would never follow Wendy's!" you shout at your screen. Yes, *you're* not going to follow Wendy's but if someone you follow boosts or replies to a Wendy's ad, you're going to see the ad in your feed.
Look at the Wendy's Threads profile, they get hundreds of replies and thousands of likes. I don't know how many boosts, Threads doesn't say. If they pay Meta to boost a post they'll get even more likes, replies, and boosts, increasing the chances of an ad reaching your feed.