It really shouldn't be a considered a "rule." Obviously the rule can't account for everyone because we all are proportioned differently. Scorning someone because they didn't use the "rule" is absurd. If it doesn't work for them, so what? Adjust the bike however you want to find the proper way to ride. As far as mounting technique goes, I'd have to say the same. However, the general, or more acceptable way to mount/dismount a bike should (and probably does) apply almost everyone.

Arguing fit is like arguing with someone because their car seat is positioned in a certain way, which is pointless. Arguing over mounting/dismounting techniques would be disapproving how someone gets in to their car. If you own a convertible you may like to jump in, rather than opening the door.