The wording in SeContainerInitializer is a bit ambiguous whether CDI Extensions should still automatically get detected if SeContainerInitializer#disableDiscovery() got called.
The spec just talks about "internal synthetic bean archive".
And the JavaDoc says "By default, the discovery is enabled so that all beans from all discovered bean archives are considered." So again also just beans.
Both wordings give the impression that CDI Extensions (which don't use scanning but java.util.ServiceLoader) might still work.

Otoh the JavaDoc contains the sentence "Moreover, it's also possible to disable the discovery completely so that only the "synthetic" bean archive is considered:" which contains "disable the discovery completely".

Both options are fine for me, but in hindsight of portability it might be wise to clarify it.