He did not say soldered. He said
on the chip - shorter data path, lower latency and higher bandwidth.
8GB of unified memory can get more data in and out in a much shorter period of time than the RAM in traditional PC or Mac with the same amount.
Homework:
https://www.makeuseof.com/what-is-unified-memory/
Theres also the argument to be made that not allowing upgrades actually lowers waste by forcing customers into thinking ahead. What are you going to do with the old SSD when it's no longer big enough? ... You "might" put it in a case and use it as an external drive. But why not just use an external drive in the first place?
Many "upgrades" spares will end up as E-waste. For example I have 12 sticks of ram sitting in a box beside me that were previous "upgrades" and will be chucked one of these days as they are not worth selling and no longer compatible or worth installing (2/4gb DDR3 SoDIMM modules). Never mind all the GPU's and HDD parts left over from previous upgrades.