I've renovated tons of bathrooms and kitchens and the poly will not really make it water proof but will give it better properties in resisting damage from water and some elasticity to prevent cranking, in any event the moss would grow just like in the video mostly by capillary action of the water being drawn up from below, mist, and splashing, I think you'll be fine. The sanded grout also gives a more natural texture and a bit of tooth for roots to grab hold not to mention the color is in the grout and not a top coat so chipping or a scratch won't require any repair to hide a color below but will make it look more authentic. Plus I think this is another area were the poly helps separate the grout from the roots so they don't burned by lime content, etc.
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