Well he is still huge in germany. Except for Michael Schumacher, probably still the biggest name.
When he turned 40 a few days ago it seemed to be a big deal and he was certainly on all the german tv channels that I*m unfortunate to have on my tv.

I'm sure if anyone can bring even more germans to pokerstars, it's Boris.
Even in other european countries, the Uk too where he has been Wimbledon commentator, he is still a bigger name than I think 99 percent of active tennis players and certainly bigger than any poker player.

So like someone said, a huge signing that can't have been cheap, unless Boris really loves poker I guess.
I know he has had huge deals in the past with big sponsors like Mercedes.