The second bloke is completely missing the nuance of the "yeah, nah". It doesn't simply mean "no". It means I've heard and acknowledged what you've said but disagree or have a different opinion. Examples
"Mate I'm fucked, want to go to kfc for brekkie?" "Yeah nah last time I went it was shit"
"Ah fuck my bad!" "Yeah nah it's all good mate, no worries"
"Trump isn't all that bad, I agree with his policies" "Yeah nah he's a pedophile rapist, you're fucked in the head mate"
You want to show me an ant jerking off an aphid to reproduce them?