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IIRC that originated on the largest Russian car-related internet bulletin board. They had a rule about posting classifieds that said "if you're selling your car, you're selling in Moscow by default, if you are not, explicitly state your locality at the very beginning and at the very end of your ad or don't complain about people from Moscow calling you".

What is the name of that board?

This is like how on certain internet forums, people talk about American conditions as if everybody is in America. Start a conversation about labor conditions and people throw around W-2 as if it's some standard thing. Or someone will give you a summary of your rights as if you are in America ("at-will states").

I remember in the early days of the internet, when you came across a postcode field, it had to be a US postcode. 90210 was the only one I knew at the time, so that got put in a lot.

Same in Poland. Warsaw is the default city.