They updated their info about this "feature" today (27 October 2026):

https://help.imdb.com/article/imdb/general-information/use-your-amazon-account-for-imdb/

> What's changing?
>
> IMDb uses Amazon's account system for sign-in and new account creation, on both
> IMDb.com and the IMDb iOS and Android apps. If you sign in with alternative methods,
> you will be prompted to migrate your account to sign in with Amazon going forward.
> In some cases, you may be automatically signed in with your Amazon account.

"In some cases"... ya, right. I bet that's "unless you've already linked your Amazon and IMDB accounts".