According to Section 12.1,

An archive which contains an extension and no beans.xml file is not a bean archive.

According to Section 11.5,

An extension is a service provider of the service javax.enterprise.inject.spi.Extension declared in META-INF/services

Conclusion:

An archive containing a class implementing javax.enterprise.inject.spi.Extension but not declaring this class in META-INF/services does not contain an extension.

So if this archive contains a class with a bean-defining annotation and no beans.xml, then it is a bean archive.

The TCK is missing a test for this scenario (archive with extension class but no service registration), it only has a test for the more obvious case of an archive with extension class and service registration.

If my interpretation is correct, then WildFly 15 has a bug, since it disqualifies any archive containing an Extension class, regardless of the service registration.

Side note:

It would be helpful for readers looking for the exact definition of extensions to move the sentence quoted above from Section 11.5 to the introductory paragraph of Section 11.