I don't think GP meant that Google decided a standard, but that the world (and in this case, Twitter) decided that whatever Google does is considered the standa

I don't think GP meant that Google decided a standard, but that the world (and in this case, Twitter) decided that whatever Google does is considered the standard. In other words, rather than checking with the standard how to indicate that content should not be cached, they simply checked whether Chrome did not cache it, and since it didn't, decided that whatever they were doing was the way to prevent caching.