Seriously, it's weird even for him. The Golden trio have taken Polyjuice Potion before, but I don't think they ever compared the way it tastes to anything they've had before. How does he know that Polyjuice Potion tastes like goblin piss if he doesn't know what goblin piss tastes like? You can't make the comparison without the knowledge of both things, which implies that, and yes, this is disturbingly gross, Alastor knows what goblin piss tastes like because he's drunk goblin piss before