The argument is that instead of focusing on the core issue that discrimination is inherently bad and needs to be fought against, white women put the focus specifically on them. This means other groups are ignored until white women have achieved their goals until a new conversation can be started.

I'm not sure whether other groups would have made progress sooner without this problem but it's a possiblity and that would make white women significant barrier.

I admit it's hard to go from that further to biggest barrier but I don't think one should discard that notion too easily. "Moderates" pushing for some bad compromise is the worse is better of politics. MLK famously considered white moderates to be a bigger issue than racist groups at one point.