Its very unlikely the TDP differences would prevent a design with an m2 SSD if Apple wanted it. TDP is 65 watts vs the 45 in the models with two relatively enormous 2.5 inch drives. This might vary depending on spec, but nowhere near enough to prevent an m2. We are talking about modern CPUs fabricated on more efficient processes vs those from 2012.

This is a choice, not a hard engineering limitation. An m2 SSD is tiny even compared to a single 2.5” drive.