I'm having trouble finding the original sequence post that mentions it, but a "fully general excuse" refers to an excuse that can be applied to anything, independently of the truth value of the thing. In this case, what I mean is that "this isn't really the important stuff" can sound reasonable even when applied to the stuff that actually is important (especially if you don't think about it too long). It follows that if you accept that as a valid excuse but don't keep an eye on your behavior, you may find yourself labeling whatever you don't want to do at the moment as "not really important" - which leads to important work not getting done.