It would be a measure of ping + cache latency. That's still different than just ping.
And is your point that the tested sites aren't popular in Russia?
>for different popular domains (google, facebook, twitter, gmail, etc)
google.com facebook.com and twitter.com are all in the top 50 sites in Russia[1]. And gmail.com is number 6000 globally[2], so it's unpopular everywhere, not just Russia. Popular sites in Russia tend to be popular elsewhere, and vice versa.
[1] https://www.similarweb.com/top-websites/russian-federation
[2] https://www.alexa.com/siteinfo/gmail.com