This is true up until it really REALLY isn't.

The main strength of microcontroller-based hobby boards (I hesitate to say "bare-metal", but something like that) is that tuning them for long operation on a small pouch cell is pretty straightforward. There is no such easy path to prolong battery life on a Raspberry Pi (not including the RPI Pico). After that, with microcontrollers, you have direct visibility into most interrupts you may need to use. You do not have that in the standard Raspbian linux distro.

They are foundationally different items, and it does not take a tremendously complicated project to reach the boundary between them. Need a robust wifi stack or to run a camera? You need something with at least an RTOS (like an ESP), or an actual operating system. Need to service a rapidly spinning rotary encoder without missing clicks or blocking other operations? You need a microcontroller.

Its certainly true that you can make a Raspberry Pi do everything an arduino can (and mostly vice versa), but in terms of what's accessible to a early-intermediate hobbyist, they are different tools for different tasks.