he is not wrong though. I've seen plenty of extremely biased players, who have lots of games on most of the factions. In fact, it is actually quite hard to point out who is unbiased.
On the other hand people who only 1-2 faction are far more likely not to know what they are talking about when it comes to units they have not even used.
So no he is not right, player who play all factions have a better understanding of the game regardless if they are biased or not. It would guess that playing 1-2 faction is actually more common of people who are extremely biased.
And no building a mortar in 4vs4 game does not "mean one lose in the long run" so again he is not right...