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.
By that logic, when you create an account for some website through your Gmail you're not creating an account.
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.
It is, they make it super easy to appropriate that into threads and will no doubt do even more over time up to and including, I suspect, creating threads accounts for all Insta users and promoting threads content to Insta users and vice versa.
Yeah like Zuck said your Instagram followers are already there - so the question is: if you gain a follower on threads, is it also a new follower on Instagram?
AFAIK, no. Your Instagram followers also aren't already there, FWIW: when you first log in it asks if you want to follow some or all of the same people you follow on Instagram; if you follow someone who isn't on Threads yet it becomes a "Pending" follow and will automatically follow them once they join. I am pretty sure the follow lists are then disjoint.
(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.
As I mentioned elsewhere, does the conversation "Okay it's just using the Insta login service just now but I'll have its own one ready next week" "nah fuckit, just ship it, it's fine like that" sound likely or indeed familiar to you?