I remember when this came out, but more recently I recall finding something indicating that this was either untrue or pointless-- for example, while the ICs can be changed out, either the SoC or board knows what SKU it is due to an e-fuse or config locked into a cryptographically-signed firmware package, and refuses to address a different amount of RAM. I can't find that reference, however.
The fixed RAM is annoying, but is done to make the product better — the packaging allows the RAM to be significantly faster than otherwise. It’s a major reason why the M-series CPU and GPU performs so well.
It’s the same reason why you don’t see slotted RAM on GPUs. The performance penalty would be great enough that nobody would buy them.
GPUs do not put their RAM in the same package as the processor itself, and it does not sacrifice a significant amount of performance.
The M-series processors' primary benefit by a huge margin is its architecture that allows a higher memory bandwidth than say SODIMMs and that benefit is independent of the choice to put the RAM in the package.
What I said: It’s the same reason why you don’t see slotted RAM on GPUs.
What matters in GPUs is predictable distances with direct soldered connections, hence why RAM chips always surround the main chip.