Some of the science is settled, certainly. Methane is a greenhouse gas; nobody expects that to change. Atmospheric methane decays primarily through a long, well-documented chain of reactions starting with oxidation by the hydroxyl radical; the carbon in the CH4 eventually ends up in a CO2 molecule. This is nothing new, and nobody expects it to change.
The precise dynamics by which CH4 interacts with hydroxyl radicals in the atmosphere is far from settled science, and nobody should be particularly surprised that there are things about the process we don't know. Not knowing some things about a process doesn't mean we can't know other things about that process.
But some people obviously do believe it means that. They do not distinguish between not knowing everything and knowing nothing. Implicitly requiring scientists to know everything before you consider science credible makes everything a matter of opinion, and all opinions more or less equally valid, at least as far is evidence is concerned. And it's easy to see the attraction: if everything is a matter of opinion you can believe whatever you find comforting. Why not believe Adam and Eve rode around on dinosaurs? After all scientists don't know everything, which means science is never "settled".
But of course settling questions with evidence is what science is all about. True, there is no science so settled it cannot be attacked; but there *is* science sufficiently settled that claims to the contrary require extraordinary evidence.