Are you paying something for this lock? I've never seen a "not guaranteed" clause on an interest rate lock. What would be the point? "We promise that we will give you this interest rate ... unless we don't feel like it when the time comes." I'd be happy to make promises like that. I can't come to a conclusion without reading the rest of the terms, but on the surface it sounds like a worthless clause. I'd be reluctant to pay anything for it.
I wouldn't necessarily go to another lender just because of that. If otherwise they seem like they're offering you a good deal, this could just be one small annoying point. If it looks like they're trying to take advantage of you at every turn and this is just one more example, yeah, go somewhere else.