It could also mean that you know he's apt for the job according to what others have said about him, but you yourself don't know anything about him.
Not at all, the phrase makes no mention (implicit or explicit) of any prestige based on opinions of other people, in fact it explicitly REJECTS that because it says "I don't know anything about him".
Unless quantified in some manner (for example, "I don't know anything about him that I didn't hear from somebody else"), "I don't know anything about the guy" means "I don't know anything about the guy".
You can't just assume that quantifiers are there, you have to take statements at least somewhat close to what they are.
lol welcome to the internet, where saying you mean someone no disrespect, infers that you do. Props to Kewkky for acknowledging that sometimes sentences mean what they look like they mean.
"No disrespect", if you say it before there's a response to the comment it talks about, is a meaningless phrase. "I think you're the most horrible being that ever exist and killing you would be a mercy, no disrespect" is still disrespectful, as is any other phrase that's disrespectful. "No disrespect" doesn't actually change anything you said, so if what you said is disrespectful, it won't change it.
The way to use, "no disrespect" is [insert comment here]->[insert response by other person]->"no disrespect, I just meant [insert explanation of comment here]".
Kewkky's comment was exactly the opposite of the plain meaning of the phrase, adding non-existent quantifiers =/= "what they look like they mean", addung non-existant quantifiers = "some random meaning which bears little to no resemblance to what they look like they mean".
I'm not trying to convince you you're a horrible person, I'm just disagreeing with a comment you made (that you never implied he was a nobody or unqualified, when you did). If you don't like that fact (and I'm pretty sure that wasn't what you meant to imply) then perhaps double-check your comments before posting them, it's a lot easier here then in speech. A good rule of thumb is if you begin or end a comment with "no offense" or anything like it, it's probably offensive.