Tablebase says that this is a draw, how is that possible?

Play along with the tablebase and see. That said, most players can't play perfectly so there are chances black could mess up and lose, especially under time considerations. With best play, it would be a draw

Tablebases are not engines. The are compiled positions and are absolutely correct. At least I have never seen any instances where a calculated tablebases is incorrect. Basically, if a material combination is in a tablebase, then that's a solved position.

The analysis engine isn't hooked into the tablebases currently, so they're not going to evaluate the position correctly.

Only thing that's problematic for white, which I can think of, is black threatening to sacrifice the rook for the bishop, but not sure how is easily that is accomplished. I guess we have to assume it's white move in the above position, otherwise Ra3+ draws immediately.

I am talking about sacrificing for the bishop not for one of the knights.

Black can exchange rook for bishop and enter a drawn ending. But of course its not easy to do that, and white can play on as he takes no risks even if he blunders two pieces.