I am ex IDF(I was not in Sayeret Matkal like him but still) and just so you know most of the combat training is not about hand-to-hand combat(some of it is for sure), the image the game created for David is totally silly too BTW(which I mentioned before).
Learning Krav Maga in a nice gym with a mattress beneath your feet has nothing to do with street fights, especially when you are going against a professional criminal with a knife stuck in your ribs.
So yeah, that part of the game is totally not realistic and it's a shitty writing on the DEVs part.
If you remember the talk before they started to train together, it was not regular army training for MC. He specifically said you won't survive in a street fight and they aimed to trained to survive in a street fight. They trained for that exact situation. Krav Maga was just a part of his training. For the knife part, as you can see in that scene he collapsed right after the fight. He was basically riding on adrenaline for his life and his loved ones. The conditions for the fight was necessary for the game, after all it is a choice based game. You choose something then that choice become important at some point of the game. You mentioned 2 weeks training with chef was not enough or something like that but in the fight sequence he did not used what chef taught him, he used her as a mental support. They did not do any grappling after all.
Our MC is a prodigy in athletics; he was scholarship worthy football player, a very good baseball player and good at parkour. This person got specific training from a man who was an ex spec ops in IDF who was a legend, who he escaped from a prison bare handed, malnourished, killed the guards while escaping. They did the training for exactly this sort of situations. You are talking about he was against professional criminal but MC was against professional bandits a few months prior and kept his cool to asses situation. So he was training to stand his ground if it's necessary against professional criminals, bandits, mobsters etc.
For the general point on realism, in these kind of media; movies, series, games, books, etc. they create their own universe. You need to accept that universe from get go, then judge according to that. You won't say Game of Thrones is unrealistic because it has dragons, magic etc. in it. So you may say these people don't exist in real life, maybe they really don't; but we have them in Chasing Sunset universe and they exactly acted as expected in that universe. Also you cannot say this game is not realistic because they cloned a human or they managed to built an R&D company, did couple of Nobel prize worthy researches and had enough time for their family. We have those people in this game. What would be unrealistic then? If MC suddenly started firing laser beams from his eyes in that fight sequence, that would have been unrealistic not because we do not have people who can fire laser beams from their eyes in real life but because this was not part of the game's universe.
I hope what I tried to say make sense.