There are two "Gmail" things here: the actual service with a web site (mail.google.com), and an android app on your phone that is called "Gmail". They are axing the "pull" path of the actual service. That path only supported POP for pulling those mails. There never was an IMAP pull path. They are telling you to read your mails of the "other" account by configuring your Gmail app to access it via IMAP. That obviously…
Yep, I have 4 gmail addresses that I set to copy and forward to my fastmail inbox, unless gmail kills that feature I am golden.