Lol, the irony...
He's not asking you to explain the quote. He's asking you to explain why you think it applies.
People in glass houses, a similar critique can be made for them. Display how that critique of him applies. In very simple terms, you said he was guilty of the same thing he is criticizing you for and he's calling you out on that accusation.
You do know we can see his posts right? There's a record of it. He's been on here a while. And from that you can gather a general SystemShock ethos and, well, he's not in any position to insinuate anything about "the type of person I am".