As a restatement of customary international law, the Montevideo Convention merely codified existing legal norms and its principles and therefore does not apply merely to the signatories, but to all subjects of international law as a whole.

It has nothing to do with being a state, except for being a restatement and codification about the internationally recognized state practice and opinio juris about what constitutes a state.

Maybe actually read the whole Wikipedia article you’re quoting from instead of just skimming the first few paragraphs.