I am not even close to a decent level in chess, therefore you have to be really skeptical to what I am going to write here. But here is my ~2000 rated opinion:
Black is not winning after 19.f4, is he?
This is kind of funny to state that black is winning here. This is just 19-th move (so people just went out of the opening), which means that the game just began and most of the previous moves were not actually a real game. Yes, white is a pawn up, but pawn up in the opening does not really counts for too much (mobility of pieces, clear vision of what to do next, preparation of the line, your mood and confidence and many more things should be counted here).
Why did he choose to draw instead?
No one can tell you except of him. But here are my thoughts:
He already lost to Anand and Anand is a current tournament leader so why gamble and stress yourself in this situation? Having a draw, you secure yourself 5 points which puts you on the same position as a leader. With few possible advantages:
- there is no more Anand anymore
- you are not stressed and got your points from the game
- you have more time to rest, prepare yourself for future games
Also, as you saw from game analysis the position is equal, but unbalanced. Playing the game, you will be really stressed and most probably after additional 1 - 1.5 hours of thoughts you will end up in a draw. Or may be win. Or lose. So why do you need this?
Look at the game Anand vs Svidler in round 7. Both have spend a lot of nerves playing really hard an unbalanced position just ending up in a draw. Approximately 2 hours of hard thought for 2 points. And here Aronian can get them basically for free.
