He didn't say that every girl is bad. As you see, he even wrote that he had 2 girls in their main raiding group, which is still good I think.
I completely agree with him, most of the girls I played with were only doing that "for fun", though we were trying to progress, like you know "collecting pets/toys/mounts etc." and then when item dropped she was like "oooohhhh, that's a lovely head for me <3" and when she didn't get it she slacked for the rest of the raid. And usually girls are the ones who don't take criticizing very well, which can cause problems in raiding. There's a difference between "most" and "EVERYONE".
And yeah, I know that there are also "boys" who don't know how to play etc.
Uhm... No, girls are the players which mostly cause dramas in the guilds.
Just in case - we have 1 girl who is good player and finished 13/13 mythic, but most of girls in my guild are only doing heroic.