It sounds like a blurring between Holy and 'slaying against...'.
It always was a little blurry, wasn't it? The absence of firm alignments like DnD has (Lawful Good, Chaotic Neutral) makes it harder in RM to define what a Holy attack should be especially damaging against. Don't get me wrong -- I'm not arguing that RM has to have DnD alignments. I'm just noting that in the absence of them, RM kind of made Holy weapons a new class of weapons, above magic but below slaying. I don't thing they are really tied (in the Rules As Written anyway) to any specific class or alignment of creatures; they are just like 'slaying-light'.
Whether it would be better to tie them somehow to specific creatures is a good question; I'm just not sure how exactly you would do that without alignments and designations of creatures as 'good' or 'evil'.
I like what RMU (and previous editions to some extent too) has done in giving Paladins spells specifically against demons and undead. So I guess you could say Holy weapons only get their bonuses versus those classes of creatures. RMU also has some spells that are clearly the Paladin channeling his/her god, and so the extra power applies to any creature (even one that is not inherently evil); these would be spells like Holy Strength.
But even RMU has to wrestle with these issues with spells like Holy Aura, which gives Holy criticals to all creatures 'of darkness' in the area of effect. Here the rules are leaving it up to the GM to decide what counts as a creature 'of darkness'.
It might be possible to further define that. I would say a creature is 'of darkness' if it:
--Is a demon or undead
--Is possessed by a demon or undead (our party had an awesome exploit for this: the party Sorcerer would possess creatures and then the party Paladin would smite them!)
--Has learned a spell from an Evil spell list
--Worships a God or other divine entity that is inimical to the caster's deity
That might not work for all settings though.