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 won't import those mails from the other account into your Gmail account.
The solution is to push. Configure whatever system handles the "other" mail address to forward the mails to your Gmail account.