The problem with the argument is that it assumes the only thing we can do to determine the validity of a claim is to evaluate how convinced certain other people

The problem with the argument is that it assumes the only thing we can do to determine the validity of a claim is to evaluate how convinced certain other people are that the claim is true. He argues that, because scientists in 1975 were (supposedly) as convinced that global cooling was real as scientists today are convinced that global warming is real, we have no other means of determining whether either of these claims are true. Presumably he would also say that, because people before the Copernican Revolution were equally convinced of the heliocentric model as people after the Copernican Revolution were convinced of the geocentric model, we have no means whatsoever of determining whether either claim is true.