With IMAP there's no particular need for fetching from third party accounts. The article itself says that IMAP is supported in the app ("You can still read and send emails from your other account within the Gmail app. This uses a standard IMAP connection, which is supported in the Gmail mobile app"). But indeed this removes altogether the possibility to fetch from third-party accounts into Gmail.
With IMAP there's no particular need for fetching from third party accounts. The article itself says that IMAP is supported in the app ("You can still read and send emails from your other account within the Gmail app. This uses a standard IMAP connection, which is supported in the Gmail mobile app"). But indeed this removes altogether the possibility to fetch from third-party accounts into Gmail.