If there are significant deviations of the Max8 with the previous 737 models, that would constitute a safety risk IMO. Banning the plane probably wouldn't be the appropriate response, but we would at least ask Boeing to develop proper simulators and training so that Pilots would be more familiar with whatever the issue is.
Going between the 737 NG and the 737 Max would not be a big change even if they were different type ratings, but going from say some turboprop to a 737 would be a big change.
If this pilot was moving up to the 737 from smaller aircraft (which nothing says he did, its just speculation), it seems somewhat likely that he transitioned from either their retired turboprops or the Q400 fleet they have.
[1]: https://en.wikipedia.org/wiki/Ethiopian_Airlines_Flight_409
[2]: https://www.bea.aero/docspa/2010/et-b100125.en/pdf/et-b10012...
[3]: https://www.isasi.org/Documents/library/technical-papers/201...