But how do we know that he aimed at signalling upper-class membership?

The alternative I'm proposing is that middle-class people will not try to deceive others about their social position (because that would never work in the long run), but they are adopting lots of signalling about their true position, in order to not get mistakenly perceived as being lower than their true position during short encounters.

I think this is consistent with common folk-wisdom about classes. I have often heard claimed that the primary concern of the lower-middle class is to distinguish themselves from working class. I have never heard it claimed that their primary concern is to pass as middle-middle class.