Indeed, however in this case as the OP has just outlined, he's presenting a US passport at the eGate. If that gets rejected then he needs to present that passport not his British one and therefore the person at the desk won't know he's a UK citizen, hence the questions.

If he says he's a UK citizen then he needs to prove it with his UK passport in which case he'll be asked why he didn't use that, so he'll say it doesn't work in the eGates which will cause the person at the desk to run that through the scanner as well.

Either way he'll be let in but I'm just highlighting a potential pitfall.