This is rather just mimicking the service loader mechanism used here to load an actual adapter builder:
https://github.com/wildfly/wildfly/blob/master/mod_cluster/extension/src/main/java/org/wildfly/extension/mod_cluster/ModClusterSubsystemAdd.java#L135
this loads its counterpart.

I.e. the subsystem is composed of the extension module and the container implementation (used to be JBoss Web and Undertow, now its just Undertow).

Sticking the undertow specifics here would introduce a circular dependency. Seems like we would have to merge the 2 modules and screw up backporting to EAP in return?