Because we're still increasing the rate at which we emit other greenhouses gases, and even if we emitted none anymore a lot of warming is already locked in.
All efforts to combat global warming about reducing or eliminating the acceleration. If one of those methods is both cheap and has a fairly high impact (which, methane reduction appears to be) then why are we focusing on "but this doesn't eliminate global warming"?
Adding renewables and nuclear to the grid won't eliminate global warming, but they slow it down. Eliminating methane appears to be a route where we can more aggressively stomp on the breaks.
The only time I've seen that argument raised is for the opposite "Because this won't fix global warming, we shouldn't do it?".
Global warming will require a multifaceted approach to be solve. No one solution will fix it, so literally any solution is going to have a detractor that says "This won't fix it, so why bother with it?"
Reducing and fixing or eliminating are not the same thing. How could they have worded the title better? "Limiting methane emissions will have an outsized impact given the cost"?
It's also true that increasing global average warming can result in reduced warming and temperatures in some geographic areas so it is important to be specific with terminology depending on which aspect of the climate change debate we're talking about. You are right that the title is unintentionally misleading in this regard.