Advertising to people currently on the Fediverse probably isn’t their goal. Neither is just joining the Fediverse. Their goal is to become the Fediverse to swallow it whole.
They also have no incentive to share anti-spam tech. They would be giving up a massive sellin g point of their instance if they did.
Additionally, Threads is already stripping its implementation of ActivityPub of some features like alt text and CWs. Worse though is that they’re removing the option for a chronological timeline, forcing their algorithm as the only option for its users. This would be fine, except it would also effect content on all instances (to an extent). Given that Threads will no doubt have a userbase larger than that of most instances, and that posts which play into its algorithm will show up for significantly more people, posts which engage with Threads’ algorithm will be boosted more often, thus showing up for more people who aren’t using Threads.
Say Threads’ algorithm really likes posts that get lots of replies. Someone writes a controversial post on mastodon.social and lots of people on Threads immediately get in arguments in the replies. This post is then promoted by Threads’ algorithm, causing it to be seen by more people on Threads. More eyes on a post means more likelihood of getting a higher number of boosts. Lots of accounts on Threads boost the post, which causes it to be seen by more people in federated instances. Again, more eyes means more likelihood of getting boosted. At this point the post is popular not just in Threads, but also in instances federated with Threads. Which means it’ll pop up more often on other instances that do not federate with Threads. Threads’ algorithm, only being implemented on Threads, extends its toxic influence out even to servers which refuse to federate with Threads.