If I didn't know better, I'd say that remark reeks of Trot prejudice, really.
I think you'll find that it's not supposed to be about 'We have so many more attendees than you'-style willy-waving. I always thought that being an activist was about effective activity, actually deciding what needs doing and then seeking out the best way to go about doing it, and then actually going out and making an effort to get whatever 'it' is done.
And your comment about 'mistakes' doesn't quite tell the whole story either, now does it? Why don't we talk, as well as about the genuine mistakes, about all the deliberate acts of destructive fuckery that the SWP have pulled, in a number of places, at a number of different time and with all manner of different people.