The idea is that the company's Zoom admin should always specify the exact list of users who are allowed to be in their Zoom account. The email domain should have no bearing. So if you sign in as bob@mycorp.com, and that is a valid corporate account with the right permissions, you are let through. If you try bob+foo@mycorp.com, it should always fail. The pattern of "oh they have a mycorp.com email so they are probably legit" was broken from the start.
This is thankfully less of an issue now since everyone is moving to SAML-based logins and SCIM provisioning.