Indeed. The Gmail app itself has been able to act as an IMAP client before, but (as it's purely client side) this mode doesn't offer all Gmail features and is quite battery and bandwidth intensive as IMAP wasn't really designed for mobile devices. With Gmailify, the IMAP synchronization happens on Google servers, which also do normal spam detection and categorization, and the phone accesses the data like any normal Gmail account.
Given the specificity of the providers they list, they are probably using their APIs instead of IMAP, which is a lot easier.
Do you think the specific e-mail providers were willing to agree on binding service-level agreements?
Or they've just whitelisted said providers after having either verified that providers IMAP implementations comply with IMAP standards, or they've whitelisted said providers because those providers were big enough that it was worth the trouble for Google to work around issues with those providers IMAP implementations.