I feel alone on this side of the room. Clay substrate is in no way a must for pumilio. Calcium supplements are a must.
Clay substrate is thought to be beneficial but does not replace the need for calcium supplements.
I personally do not use any clay based substrate. Although I have no objections to it, I have no interest in it. I value it, but I also value the substrate that I use. I have raised a lot of pumilio froglets to adult hood with virtual no loss. I do pull the froglets as soon as I find them and raise them in smaller tanks where I can better control their food.
My growout tanks are very well seeded with springtails which I feed with Repashy Bug Burger
(available at many of our sponsor (with the exception of myself right now - an oversight)), which contains calcium. Springtails eat the bug burger, gain calcium, and the pumilio froglets eat the springtails. I also use bug burger in all of my breeding tanks as feeding stations, instead of the common fruit.
Don't get me wrong, clay substrate has it's place and is a great substrate.