The OP delay loop is for MCU/Arduino where embedded devs typically have all control of the PLLs and know exact CPU timings. You can see that the other part of the bootloader example fixes all related clocks manually. [1]
These MCUs are designed for hard realtime systems so the silicon design is required to have all timings well-known in the datasheets and required to be deterministic. The core (Cortex-M0) design removes nondeterministic aspects of the CPU so there are neither caches nor branch predictions in it. So the busy loops have deterministic timings.
Of course it won't be trivial in MPU/Linux spaces as you mention.
Lots of programming practices are quite different between MPU and MCU worlds. I personally find it interesting that many people are talking about MPU/Linux/x86 here. It's good to learn about them :)
These MCUs are designed for hard realtime systems so the silicon design is required to have all timings well-known in the datasheets and required to be deterministic. The core (Cortex-M0) design removes nondeterministic aspects of the CPU so there are neither caches nor branch predictions in it. So the busy loops have deterministic timings.
Of course it won't be trivial in MPU/Linux spaces as you mention.
Lots of programming practices are quite different between MPU and MCU worlds. I personally find it interesting that many people are talking about MPU/Linux/x86 here. It's good to learn about them :)
[1] https://github.com/arduino/ArduinoCore-samd/blob/master/boot...