I would imagine that's because russia wouldn't have made a big difference in support of brexit, and would look like the bad guys if they used threats to promote remain. They didn't really have anything to gain by being outspoken on the issue.

The point about the USA stands. Its morbidly ironic that leader of the USA would threaten anyone outside the US for trying to declare independence from a union that was threatening their way of life. And this dude is a goddamn constitutional "scholar."