I don't know if eleven guys can pack enough explosives and detonate them simultaneously to collapse the PATH Tube. But their goal was quite nuanced. Far from trying to flood lower Manhattan, which as most know is above sea level, they sought to flood the PATH and by connection the nexus of subways throughout lower Manhattan. The corrosive effects of salt water would have damaged the Subway and PATH systems for months, maybe years. As New Yorkers discovered again last December during the TWU strike, NYC without its subway system is an economic shell.
I don't know if they could have made it work, but their plan was and is anything but dumb.