Respectfully, it's not the people size that counts in this case. It's the size of messages, the size of Twitter accounts they used, which to me seems convincing. They do a decent job of coming to conclusions based on the messages themselves, combined with what people said.
Also, I don't see why the people sample size is a problem. Because they never made any claims about the demographics of the political spectrum they presented. They merely categorized them and defined them.

Most of their conclusions were not based on statistics, but rather psychological, social, political speculations which also they acknowledge. Sample size and statistics would only make their speculations stronger, as it closely matches with the behavior and conclusions in the west.