I don't remember about this specialist, but:
1) It seems that there are specialist skills that don't correspond to factions;
2) There are specialists that are only embedded in the executable file.

It's a pity if everything is embedded in the executable file - I have an idea to equalize all specialists - it's all a matter of numbers - for example, if the Mercenary's specialization is made like this*, then where is the imbalance, because you might not even find undead on the map, and if you do, you are unlikely to find squads-legions in multiplayer
* 10%+(1*hero_level)% of the initial squad size (in this case, the advantage of the Mercenary will primarily be that it will be possible to break through without a fight to mines/locations/artifacts, but even then not always, but only if you're lucky and an undead squad appears (the probability is somewhere around 12.5%)).