Yeah, for some reason people say "you can't prove a negative" but mean "you can't prove a universal". Any positive assertion can be trivially restated as a negative assertion, so it would be impossible to prove anything if it were impossible to probe a negative. Any negative assertion can likewise be trivially restated as a positive assertion.

Hell, "you can't prove a negative" is itself a negative -- so it's tautologically impossible to prove that you can't prove a negative (either because it's false or because it's true).