Really not the same beasts. The Cortex M4 microcontroller is self-contained with every peripheral it needs (including BT and wireless stack) and its power consumption is very small compared to a SoC.
Also, it's not booting Linux at all. For these simplish one-purpose applications, you don't want the overhead and it probably would need more than the 24k or RAM onboard the nRF52810.
They could be using Zephyr though as it supports the Cortex M4 (it's a linux-like RTOS that can be heavily customised).
No, short term and long term availability of a part in volume has always been a part of circuit design. The current situation has been a unique extreme of this, but buying a million count of an electronic part hasn’t ever been an arbitrary off the shelf affair. How many a distributor has for immediate shipping, the lead time for large quantities, and the end of life plans were always a consideration. Before though the problems you were trying to avoid were more like not delaying production for six weeks because a part was out of stock or whether you could keep building this board for the next ten years.
I would agree for million (depending on market segment, you could be talking straight with mfg then). But we were talking about 1k, a tiny quantity as far as production rounds go.
It's a recent thing on a world scale. But it's always been an issue and applies to anything you buy - the more sophisticated, the fewer items are stocked in warehouses. It's not that they wouldn't be available, it's that once you start ordering hundreds, the lead time will change from days to weeks at some point.
Hundreds is really a loose retail quantity if we talk about microcontrollers, the first real price break point is typically 1000pcs. Smaller quantities are cut and priced by distributors themselves.
In normal times, most products not marked as EOL would be available in tens of thousands with short lead times.
Also, it's not booting Linux at all. For these simplish one-purpose applications, you don't want the overhead and it probably would need more than the 24k or RAM onboard the nRF52810.
They could be using Zephyr though as it supports the Cortex M4 (it's a linux-like RTOS that can be heavily customised).