The average user doesn’t know or care about the “perks” of non-Facebook Mastodon instances that Eugene is talking about. They will go with the service with the most name recognition every time, rather than trust an independent, small-time instance operator.
Threads advertises itself as "interoperable with the Fediverse", which will fuel curiosity in some users. And Mastodon isn't only small-time operated instances. Creators of Vivaldi web browser created their Mastodon instance and bundle it with their browser account. Mozilla is preparing to do the same.Medium and Flipboard are another examples.
Nobody on a Lemmy instance is able to follow accounts (like on Mastodon or /kbin). Thus, Lemmy will not fetch anything from Mastodon unless written specifically to a threadiverse community (and the community being CCed). Because of this, Lemmy instance are less harmful, than (potentially) any microblogging server (be it Mastodon, Pleroma, Soapbox, key, etc.)
This is not an exhaustive list. For example, Instagram Threads profiles are available from kbin.social, which is not listed here, though.