The CMB is the afterglow of the universe becoming transparent about 380,000 years after the Big Bang. It tells us nothing about t = 0 or whether there was anything before.

Correct. The microwave background confirms a hot, dense early state and rules out steady-state models; it does not reach the beginning itself. Start here means the start of the current expansion phase. The stronger claim about an absolute beginning rests on the singularity theorems and on the Borde-Guth-Vilenkin theorem, which the Atheism case cites, and even those leave the quantum-gravity regime open. What the CMB settles is that the universe has a history and a finite age, which is what the older eternal-universe picture denied.