From the linked page: "From ὄλυμπος (olympos), a proto-Hellenic word for mountain"
It would be interesting to learn about the source of this information.
I tried to find another source giving "olympos, Proto-Hellenic/Proto-Greek for 'mountain'" on the Internet, without success.
Anyway, why should the *name* for a mountain be just "mountain"? I think that this may happen out of misunderstandings between two people with different languages, if one group assumes that the common-day word of another group is a name and starts to treat it as such, but is this seen as such a case?
Rene
www.AboutNames.ch