Well, no. Foster will grant that. But that isn't what he meant, and this scruffy rando knows it. (Or at least, Foster is projecting onto this scruffy rando and therefore believes he knows it. Fine lines.)

It's why he looks completely unimpressed with that response until Lambert says something that finally makes him perk up.

"Oh, you noticed!"

He's bright-eyed suddenly, if not beaming. He doesn't think to question how Lambert deduced this--Foster assumes that his defect is something implicitly obvious to others--as visible as, say, the colour of his hair.

"You're mostly right! It's a disease. Don't worry, it's not contagious. It's unique--heh--it's unique to me. It's only natural to be repulsed, though. Feel free to avoid me, or spit on me, or whatever it is you feel like doing!"

He pauses, and then--as though perhaps this is reassuring:

"I am definitely all here, it's more like... hm, this is all there is. Basically nothing!"