equip specific creature type isn't the only solution, and would in fact still cause issues. You could print a sword and say equips to humans and thats all well and good but what if you print an armless human? and why can't elves use swords? its still a big snakes in boots problem, and ends up being even more inconsistent.
I think a better alternative if you wanted equipment to be handled in a way that is flavourfully consistent would be to have equipment types and have creatures have equipment slots. That way you could equip gloves to anything with hands, for example. If the types were vaguer there would be more contradictions but it would be easier to communicate the mechanic. It works better than equip specific creature type anyway. I'm sure if a design team could decide on an even better execution if they thought about it for a while.
Good point, although I'd like to point out that making equipment slots might make the cards too busy. I recently saw a card from another game (through Steve Belledin's blog
here) which I couldn't help but notice had like four times the number of shorthand icons on it, leaving me utterly confused as to what it does.