Your conclusion is correct but to be clear the memory is not "fused." It's soldered close to the main processor. Not even a Package-on-Package (two story) configuration.
I think by fuse I mean't its stuck on to the SOC module, not part of the SOC as I may have worded. While you could maybe still add NANDs later in the manufacturing process, it's probably not easy, especially if you need more NANDs and a larger module which might cause more design problems. The NAND is closer cause the controller is in the SOC. So the memory controller probably would also change with higher memory sizes which would mean this cannot be a last minute change.
See photo without heatspreader here: https://wccftech.com/apple-m2-ultra-soc-delidded-package-siz...