One of the marked characteristics of English as a language is that (almost?) any part of speech may be used as any other part of speech. In this case, the noun "summit" has been used as a verb, either transitive or intransitive, in mountaineering or in international politics.   

"Summited" is a word. I recognize that you don't like that word, but it is a word. I'm not fond of it, myself, but it's a conventional sound/writing symbol which is present and used on a fairly common basis. That's what a word is, and even without the "fairly common" usage--that is, if that symbol had just been invented ten minutes ago by one individual writer, for a one-time use, it's still a word. Maybe an untraditional word, maybe an ugly word, maybe an awkward word, maybe a colloquial or slangy or informal word, but a word nonetheless.

This reminds me of the saying I used to hear in perhaps second grade and after, "'Ain't' ain't a word." Or "'Ain't' ain't in the dictionary." Of course "ain't" is a word, and it makes no difference whether it's in "the dictionary". One could, in those days, find a particular dictionary that didn't list it, and you might find such even today, but that doesn't change the fact that it's a well-established, common word in the language.

You don't like "summited"? Don't use it.

Dave Oesterreich