I've tested this out and the brake lights turn on when using regen, even at low levels at slow speeds. So, doubt this is a real issue.
In "L" gear in my Chevy Volt the brake lights do not come on under full regen, however, nor do they in any manual transmission ICE car I've ever driven (quite a few) during downshifts... so, I think this is a Tesla specific safety feature that I appreciate.