The level shifter isn't for protecting the Pi. It is because the serial data input on a 5V powered WS2812 isn't technically compatible with a 3.3V GPIO.
The WS2812 datasheet gives the voltage thresholds it considers to be input low and input high.
Input low is < 0.3 x Vdd.
Input high is > 0.7 x Vdd.
Most setups use 5V to power the WS2812 strings so input low is anything < 1.5V and input high is anything > 3.5V. The problem is that the Pi GPIOs operate at 3.3V so a high output is in the undefined region between input low and input high of the WS2812.
If you're doing this for your own personal projects then just try it without the level shifter and if it works then it is fine. However, if you're intending to sell this then you should do it properly. WorldSemi or one of their cloner may make changes to the internal circuitry that pushes the actual threshold up to 3.5V or just normal manufacturing variation affects it. They're still within the published specifications but suddenly your Pi won't work with a new batch of LEDs.