You Make the Call

I had a very strange ruling tonight at Mirage that may have been the correct ruling but not one I've ever seen applied the way it was. Here's the situation: we're playing 4-handed limit hold'em. Preflop action folds to the small blind. He raises. I 3-bet, he 4-bets, I 5-bet. Then the small blind tries to put in a sixth bet and here's where I stopped and asked for a ruling.

All previous applications of the rule that I've ever seen always specified that unlimited raising is only allowed if a street started heads-up. Since there were four players at the start of this street (the preflop action), the "bet plus 4 raises" cap would apply. However the floor ruled that because we were heads-up the action could proceed for as many raises as we wanted.

Doing some digging in Robert's Rules, here's what I found:

Unlimited raising is allowed in heads-up play. This applies any time the action becomes heads-up before the raising has been capped. Once the raising is capped on a betting round, it cannot be uncapped by a subsequent fold that leaves two players heads-up.

It seems that Robert's Rules agree with the floor ruling I received. What experiences have other LHE players had? And thinking things through logically... does this make sense?