"Wallflower" has the connotation of being almost decorative, for show. Basically, he's calling out that they were active participants important to the discussion instead of observers or accessories that the discussion could have been had without. It's not so much that they weren't shy, it's that they materially impacted the meeting.
I think this was chosen to contrast it to an alternative method of diversity where many women are included but they are not as important. His thesis is not to optimize for "numbers of bodies" equal representation, but equal representation of influence.
The definition says that you might refrain from either shyness (your own preference not to participate) or unpopularity (others' preference that you do not participate). I think the latter was more what was meant here.
I'm extremely shy in certain situations (for instance, I hate introducing myself to people at parties, bars, etc.), but if I'm in a meeting for a group I'm involved with, I will absolutely say as much as I'm allowed to say.