If well established practices and fact are disputed by one party, that doesn't make it objective to abandon them. If non-partisan means splitting the difference between the major parties, then there would be an incentive for each party to tack as far away from center in order to sway the GAO.
https://en.m.wikipedia.org/wiki/Nonpartisanism
If the entire Trump administration disagrees with it, I think it it's fair to say that his political party is not on board.
The article/report is leveraging one Republican (moderate Susan Collins) to label it as nonpartisan.
Partisan means pushing a political agenda based on ideology or party. NOT that a particular party agrees or disagrees with them.
Whether or not the changing climate is anthropogenic is somewhat less certain, (and more politically contentious), but we ought to be able to discuss the effects of these absent the cause without calling a report partisan.
> in most cases, nonpartisan refers specifically to political party connections rather than being the strict antonym of "partisan".