Yeah, I agree with this. Everything indicates that Anduin has the full Alliance Priest training: He learned both Holy and Shadow sorcery, like the playable Priest, but his nature (gotta help others!) favors the Holy spells.
Being threatened and stressed in the Maw may change his problem-solving method tho. A cornered Priest will sooner use Shadow magic before succumbing. And in the end, both magics are powered by his Faith, so he would be just as strong as Shadow caster.