Here is a really good article that recaps ECOC:
https://www.gazettabyte.com/home/2024/10/11/ecoc-2024-industry-reflections.html
Sounds like Mitsubitshi isn't the only one with a 400G EML. Lumentum and other companies privately showcased them.
Also this article makes it apparent that 400G using PAM6 is not desirable.