The total amount of xp needed for lvl 0 - 10 is 63700. That means he have gotten less than 637xp per hour played. Wich seems almost impossible tbh factor 50-70% scavrun into that math and you are at 1.000 - 1.400ish which equals to like one full berkut and 1-2 scav kills each...
IF they offered a quasi single player version of the game. Frankly, when I did play a lot, several wipes ago, and got to level 34 it was ...ehhh. Why cause it was a massive GRIND because most of the time I just kept rerunning mission over and over when I was level 5 dyin...