Those blocks are no more valid than pork is to a Jew. Users make a choice of what to follow. Miners make a choice to serve all or some. There becomes a size of the minority that it becomes more profitable to miner to serve all than only some.
The fact you think there is any kind of sybil attack in a UASF shows you are clearly misled or little understanding of how a UASF works. A UASF does not use any kind of user count metrics. Users simply agree to it or not.