A skateboard can certainly be used as a weapon. But it is not a weapon per se. The sentence "The person was armed" implies armed with a weapon.

You can use the term 'armed' for things that aren't weapons of course. "He came at them armed with a skateboard". "She came to the lecture armed with knowledge". "He confronted his ex, armed with a restraining order". But you wouldn't describe such a person as armed without modifiers.

I admit it's semantics and I my wording was too strong. Reasonable people can disagree, though I think they are wrong. Regardless though at least one of the victims was still unarmed even without that caveat.