If a lot of people self-select themselves to align with techbro culture, techbro culture is mainly in the Silicon Valley but not only there, and they think, for example, owning a Mac is how you signal belonging to that group (as in, Macs are a modern day Veblen good), then people who think they are (or wish they were) part of the techbro culture, no matter their location, will buy Macs.

So, the claim being given by people quoting the SO survey is: SO survey responders are not mainly made up of people who self-identify as techbros or have had their purchasing decisions (such as buying a Mac) greatly influenced by techbro culture.

I do not know of a way to argue that position. You can state that, literally, techbros live outside of the US, which is obviously true without quoting this survey; it however, does not mean the Silicon Valley doesn't act as an echo chamber, and doesn't have great effect on the tech industry worldwide.

What I would more likely want to see is, of that 21% of America, how many live in either the Silicon Valley or the Seattle/Redmond region. As in, how many are hailing from the Chicago or New York or Dallas metros, or non-metro regions entirely.

I'm guessing that, still, many would have responded that they live in either the SV or the northwest, greatly more-so than elsewhere in the US.