What does that have to do with anything? Mastodon is explicitly setup to allow all user data to be harvested. What Threads supports or doesn't support in the end has no bearing on Mastodon having all user data public.
This, but unironically. If you are uncomfortable with the idea of a zero-marginal-utility medium distributing your content without your consent, you probably shouldn't make and share digital copies of your work.
I think you would agree that harvesting that information is far easier if it's being literally POST'd to your servers (which ActivityPub does) than if you're going out to scrape them, no? It's the same principle with defederation; either they're going to scrape all the data, or the data is going to be literally sent to their platform.
The point is, the idea that "don't use threads" solves the problem being presented (your data being harvested), is wrong.
What does that have to do with anything? Mastodon is explicitly setup to allow all user data to be harvested. What Threads supports or doesn't support in the end has no bearing on Mastodon having all user data public.