Yes the thicker bar is only for turbo/high-powered micras. It's definitely too stiff for a low-powered car. The stiffer bar is designed to compensate (provide resistance against) lots of power when the car is cornering under power (heaps more forces generated compared to a low powered / NA car).
The thicker bar also makes the car heaps more twitchy and prone to the tail flying out and losing control..
Just had a look at their site. I think it's a typo, if you go into the details for each bar you should see reference to 27mm for the BNR29Z.
Next to where it says "note", but I'd call them if I were you.