Yes, I am more than aware that the fediverse can see Threads profiles and posts. However, adding to what T said:
Federation is an optional feature, and does not account for profiles which have it disabled. So those simply won’t pop up on Mastodon.
Threads has not been received well by a huge chunk of the fediverse, including myself, due to Meta essentially collecting data from users outside of Threads without their explicit consent. As such, many instances have defederated from Threads.
Federation on Threads is only available in certain regions. To be exact, if you’re a Threads user, you cannot enable federation unless you live in the US, Canada, or Japan.
It may be an option for you, but it really depends on the instance your account is in, and who do you want to follow. Shoelace, on the other hand, would only be limited by the capabilities from unauthenticated API endpoints Meta provides, but any public post or profile can be opened.