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 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.
Heads-up to people trying this: Gmail will often put forwarded email into spam. Be careful especially in the beginning to check your spam folder. They may also reject the mail as spam, esp if your volume is large.
IIUC it’s hard to make forwarding to play nicely with DKIM and spf. There’s some disagreement on how to handle it. (I’m being purposefully vague as I did interact with folks handling this on the google side and don’t want to cause them trouble for helping me out).
There are also rate limits on Google side for incoming mails. By just forwarding four domains to my Gmail I used to hit them quite often. Then 2 years ogo, I stopped forwarding and switched to the now discontinued Gmail fetching domain mails over POP...
free email forwarding was one of the most useful features of Google Domains before selling... since the selloff, I've configured the domains to use Cloudflare which offers the same feature relatively transparently. I haven't seen too many issues with non-spam going into my spam, even relayed mail.
Aside: I do have a dedicated ip/vm for mailu setup, and will likely switch to using a vanity email as my catch-all instead of gmail soon enough. It's kind of sad how generally bad email has become at this point. Will also likely start playing with a few different self-hosted webmail clients, I'd considered and played with Nextcloud, just not sure how much I care for it or not.
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.