That makes sense considering that QLC (2^4 charge levels) only stores 33% more data per cell than TLC (2^3 charge levels). Therefore at best it can only be 33% cheaper/bigger.
Quad bits to triple bits is 4/3=1.33.
Taking your way of counting to the extreme: if I have a cell that can handle 1 million bits, I can't just add an extra bit of storage for another 1 million bits of storage. I have to actually add the full megabit of storage.
Yes, you are right. I mixed it up with the change from SLC to MLC where 1 extra bit was double the density.