Actually it does make sense if you limit the words to strict/literal definitions. (Also, the querent is not really using ‘still’ in a temporal sense I think although I know I see s/he put down ‘time’ in the tag line.)
What he or she is asking is this: Can something which can’t be grasped in the first place ever be slippery? Does slipperiness not imply some degree of graspability, even if that degree is minimal? If it were never graspable in the first place, then it would simply be ungraspable (and so decidedly not slippery).
It’s clearer in the example in the details. If we use dull only to qualify the sharpness of a thing (and not characterize a stupid person, say) then we can see it doesn’t quite apply to things that have no edges ‘to begin with’ – can a sphere be dull even though it has no edge, has nothing with a measurable sharpness? Can something which was never sharp (or never intended to be sharp) be called ‘dull’? What defines the word dull is after all an absence of sharpness. It is not technically incorrect to say a sphere is dull, but it is redundant (and mostly meaningless).
This is the question and the answer depends on how you look at the words.
My first quip addressed the problem of denotation and connotation that is at the center of the question. E.g. I say ‘I’m so fucked’ all the time (in fact I think it’s in my profile, not sure), but not because my orifices are being penetrated or violated, sadly. More because that’s the only thing I can think to say when facing the existential angst and/or hopelessness that pervades daily life. (Which begs the question, if I feel so persistently hopeless that it becomes the default emotion or sensation, do I then possess impunity to being existentially ‘fucked’? What kind of emotional or psychological orifice-closing occurs after so many years of hopelessness?) Which is to say that a wall cannot be fucked in the common and strictly literal sense. But like every word, ‘fuck(ed)’ owns a ton of meanings and actually was probably a bad example because it’s so versatile.
Can we bury a person who is already buried? Only if we dig him up first. But then he’s not buried anymore, so we’re not burying a buried person. We’re burying an unburied person (i.e. ‘a person presently not buried’, rather than the possible ‘a person previously not buried’ reading of unburied).
I hope this clarifies rather than obfuscates (but then as we just learned one cannot probably obfuscate something which is already obfuscated or unclear; he can only obfuscate further, which of course is a totally different verb than obfuscate).