Email is a collection of different protocols: SMTP, IMAP, POP are all different protocols that serve different purposes. Oversimplifying a bit, but - SMTP is used to exchange messages between mail servers. IMAP and POP are used to synchronize mail between a mail server and a mail client.

In other words, they absolutely can shut down IMAP and POP but still send/receive for gmail.com addresses. The main reason reason they wouldn’t do this is that their larger clients on Google Workspace need that functionality, but it’s the type of thing you might imagine them taking away from the unpaid version of Gmail to nudge companies over to Workspace.