> Is AbstractDocumentBuilder actually an SPI type, i.e. are we expecting others to extend it and build their own document builder types

Not in my world, but Sanne Grinovero might know more

> Regarding the getter, if it is not intended to be used by integrators, who is the intended user then? Maybe it just could be removed? Or if required internally, moved to an internally-used extension of AbstractDocumentBuilder?

It is for internal use.