It's not creating a "shadow account", it is literally the same account. Threads is just a different frontend for what already existed with Instagram. And Meta would've been stupid, if they wouldn't have use what they already have.
Oh for sure, but it does make me wonder how accurate the sign up reports are.
Does Threads report people who actually sign up and "claim" their "shadow accounts," or does it count actual accounts and "shadow accounts?" The former is definitely a more accurate count, while the latter is basically numbers padding.
My understanding, it is reporting people who specifically elect to sign up for Threads using their Instagram account. On instagram profiles, they have been showing a badge with their Threads subscriber number that you only get when you elect to join Threads. This increases sequentially.
The highest number on the badge should give a good indication of how many Instagram users at least “claimed” their Threads account.