The phrase let alone is used to emphasize that if other more significant or pressing things are not possible, a lesser thing certainly is not or cannot either which is contradicted in this context where Shaun and Mike won't even look at the each other but were speaking to each other during dinner.
Consider the staetement "Shaun and Mike had not eveb looked at each other but also talked to one another during dinner.". Based on this statement, is "Shaun and mike had not even looked at each other let alone talk to one another during dinner." a true statement? Yes/no/unknown