For example, I feel very sure there's no such thing as blue weight. The expression "blue weight" is syntactically legal, but that doesn't mean it refers to anything. "Time travel" appears to be in the same category. Syntactically legal, but you find that if you try to express precisely what you mean by it, you can't. It's effectively a type error, like asking for the square root of a stream.

Incoherence as a concept is a different sort of impossible from mere technological "impossibility," like e.g. a pill you could take that would let you survive in outer space without a suit. You can say what that means, even if you can't do it. I have never heard a coherent explanation of what "time travel" could even refer to.