Q is saying that he didn’t think of that as a thing to do with a Dementor and should have. One possible reason for his “very odd smile” is that he did already think of it as a thing to do with a horcrux. I think it’s reasonable to call this a hint that maybe there is a Voldemort horcrux buried deep underground. (Certainly far from a proof that there is, though.)
Q is saying that he didn’t think of that as a thing to do with a Dementor and should have. One possible reason for his “very odd smile” is that he did already think of it as a thing to do with a horcrux. I think it’s reasonable to call this a hint that maybe there is a Voldemort horcrux buried deep underground. (Certainly far from a proof that there is, though.)