I'm from Wisconsin.
When I bought my first lightweight TT, Wisconsin DMV told me the same thing, it doesn't require a plate.
Well I was going to hit the road with the TT traveling south and most certainly was not in the mood to be stopped by every cop in every county I drove thru for no plate.
You DO have the option to buy a plate for it.
The Wisconsin DMV offers this option.
IMHO, JMHO I would buy the plate versus getting stopped every day as you travel.
I bought a plate for the TT even though it was not required.