[RPG] Can Orcus use Multiattack with any melee weapon

This is kind of a fussy question, but I was reading through the stat blocks for some of the demon lords in D&D 5e today and came across a seeming edge case that I hadn't thought of before.

Orcus has a Multiattack feature, like many monsters, and this multiattack is restricted to a specific kind of attack (melee with the Wand of Orcus):

Multiattack. Orcus makes two Wand of Orcus attacks.

Usually the multiattack restrictions limit using particularly effective attacks or attacks which don't make as much sense as part of a single attack sequence (so it may restrict saving throws, or be a claw-bite combo, etc.).

But for Orcus it's not clear to me if he should have a multiattack option with any melee weapon in his hands, or just the Wand or Orcus. My thinking is that the Wand itself doesn't grant multiattack to anyone wielding it, Orcus' power isn't derived from the Wand, and Orcus is himself a fundamentally difficult adversary to deal with.

On the other hand, Orcus' stat block makes it clear that he usually has the Wand in his possession and in rare instances sends it elsewhere. It could be that the multiattack is a special feature only Orcus gets which, along with is other abilities, would make him loath to part with it. I'm also aware of the general rule that things in 5e do what they say rather than what they imply. It's also easily possible that campaign circumstances are meant to alter the challenge of a fight with Orcus.

At my own table I would probably rule that Orcus can make a multiattack with any melee weapon he holds (so his tail attack is never an option to include), and if unarmed could make two unarmed attacks. But I'm curious if there is an official ruling anywhere indicating the RAI of Orcus' multiattack, or if there are other considerations I've not thought of on this point which point to restricting this to the Wand:

Could Orcus make his multiattack with any melee weapon, or is it restricted only to the Wand of Orcus if he's wielding it?