So, if the accounts are linked closely to each other, and an Instagram account is a necessity for a Threads "account", are those really "signups" or more like... logins into another view of the same service? These numbers seem overblown.
Threads still takes you through the expected signup loop of adding bio/description, finding follows and what not. It's just missing an email verification because it goes through IG.
Which seems like a misstep because now my "Threads" account has 10% of the followers of my IG/Twitter.
(There actually seems to be a bug in this whole scheme btw wherein there is a window during the onboarding where your account is Private and if people follow you during that moment they become Requested follows you have to Confirm even if your account is immediately public and is swarmed with hundred of followers. I thankfully noticed and dug out this stragglers.)
The users and accounts are definitely otherwise the same, though: you can't even AFAIK sign up in the Threads app... you can only log in using an existing Instagram account that is already logged in on that same device, which seems to just use shared keychain access to the login tokens from the other app. I had an old version of Instagram installed and realized I couldn't even log in to Threads until I upgraded it.
Apparently this also spams them with an invite, which may be unpopular.