The version .409 which you are talking about, that is still available on Asus support page and that version is Android 10 beta version for Max Pro M2, released on 31st January, 2020. Since its a Beta version, its very much expected to have numerous number of bugs. Stable version gets released only after receiving information of those bugs from the beta testers.
Now about .073 version, which is the last firmware version for Pie users, which was released in March, 2020 and you CANNOT install the .073 version if you are on Android 10 beta firmware. Android 10 beta version .409 doesn't support widevine L1 certification either. If you want to install .073 Pie version, then you first have to downgrade from Android 10 beta version to .071 Pie version by using the only "Downgrade version" available on Asus support page. You have to follow all steps very carefully before going to downgrade. Steps are mentioned clearly at that "Downgrade version" details.
Only after downgrading to the .071 Pie version, you would be able to install the .073 version.
P.S. - .073 version does support widevine L1 certification.
