The reverse polarity protection on the board protects everything except the FETs and the blade.
The reason for this is that the FETs carry a lot of power, so reverse polarity protection would be very bulky. However, for the FETs to carry current backwards, the current also has to go through your blade. Dmitry pointed out a while ago that WS2813 based blades have reverse polarity protection in them, but most other neopixels do not. Of course, old-fashioned LEDs have reverse polarity protection since they are diodes.
The V3 throws another caveat into the mix: The reverse polarity protection won’t really work while charging. So if you plug in your battery backwards and try to charge it, you end up with a very efficient battery drainer…