At least so far the IPs do seem to belong to Yandex. It isn't as hopeless as, say, the MJ12bot which can come from absolutely anywhere-- including a good many blocked server farms. And, conversely, you don't see a lot of Yandexbot spoofers. Fake yandsearch* in referers, yes. Fake UAs, not so much.
* I did some quickie experimenting. The fake referer isn't any different from a real one. Except that currently mine all end in &lr=213. I don't know what "lr" is, only that it doesn't correspond to "cd".