Fair point. The 100% silicone is the best option. Honestly never paid attention to the fact it was a mix. Alex Flex is commonly available, "100% waterproof" (when not submerged), and often cheaper though.

Since the area in question is above the tubs water line it would do the job fine but you are right, 100% silicone is better so long as you don't want/need to paint the area.