How in the world did somebody with decades of mainframe experience wind up in an interview for a SaaS tech lead? On the surface, it sounds like there was a fairly obvious misalignment between the candidate's experience and the experience required by the role.

Somebody like this almost certainly should have been filtered out at the resume review stage or in a phone screen. There is no reason a company should be investing staff time in an in-person interview before it has a clear understanding about a candidate's experience.