Actually, people did dispute that. Most notably, a University of California professor wrote a paper arguing that "because Senator John McCain was not a citizen at birth, he is not a nnatural born Citizen' and thus is not 'eligible to the Office of President' under the Constitution."
It wasn't a bill, anyway, it was just a resolution "recognizing that John Sidney McCain, III, is a natural born citizen." In other words, they weren't claiming that the "bill" made him eligible, merely that they were recognizing his eligibility.
I believe it was registered as a ‘sense of the senate’ inclusion in the record. The vermin in the DC fen have tried on several occasions to circumvent the Constitutional requirement on president and vice president eligibility, and every time it is an effort to avoid the prescribed method for changing the Constitution.