Yes, that is true. They are definitely heavy even though the plants are small. I wouldn't do it with bigger plants, I ain't that young anymore
I am thinking if this works and I continue to use it, a trial with 50% perlite might be a good idea to keep the weight down. Mostly I am playing around with this in order to, hopefully, get plants to root faster and provide a good over winter mix that won't promote rot. Your experience with pool filter sand does not bode well for that though.
I might pull one of them to see what's going on underneath, just to make sure the same thing isn't going on for my plants.