The scenario is that the 512gb M3 Ultra was validated for the Mac Studio, and in volume production for their servers, but a business decision was made to not offer more than a 256gb SKU for Mac Studio.

I don't think this happened, but it's absolutely not "literally impossible". Engineering takes time, artificial segmentation can be changed much more quickly.

From “internal only” to “delivered to customers” in 6 weeks is literally impossible.

This change is mostly just using higher density ICs on the assembly line and printing different box art with a SKU change. It does not take much time, especially if they had planned it as a possible product just in case management changed its mind.