It sounds insane to me too, that's why I was wondering.
At the moment I'm not really playing for money yet, just some freerolls
and play money, so it's not an urgent matter, but looking around on the interwebz got me thinking.
I saw some crazy stories that sound too absurd to believe, but absurdity is sometimes normal with the law.
How you say it, it sounds indeed difficult for the tax service to know and to calculate, especially because they cannot just see what's on our bank accounts here, only if they are suspicious they can request it from the bank.