A smartphone that runs Android, launches Debian, and dual-boots Windows 11

It is hard to associate in the same sentence the word "powerful" with a quadruple Cortex-A53 CPU.

Cortex-A78 is in a totally different class of "powerful", when compared with Cortex-A53. Cortex-A53 has not been designed as a top performer among Arm CPUs, but only as a low-power core. Even at its launch, in 2012, 14 years ago, Cortex-A53 was much slower than the older big Arm cores, like Cortex-A15 from 2010. I am aware of this from direct experience, because a decade ago I have developed software on several SBCs with Cortex-A53, including Raspberry Pi, and also on an ODROID model with Cortex-A15, which ran circles around them, and unlike them it delivered a passable desktop experience.

On the other hand, at its launch in 2020, Cortex-A78 was faster than any older Arm CPU cores. It was then surpassed by the Cortex-X1 launched simultaneously with it, and then by the Apple M1, launched later that year.

Even if some enthusiasts have used old Raspberry Pi and similar SBCs with Cortex-A53 as PCs, that was really not wise as they were too slow for a comfortable use and there were faster alternatives with a similar price (after you added the cost of all required peripherals).

It's more expensive and not sufficiently more powerful: I would also have to pay customs duties to import it into Serbia. The above are actual devices I own or have owned, it's natural I did not get all of them (including like PinePhone Pro).

While I partially agree with what it says at your link, about specs depending on the context, I am also well aware about the actual performance of quadruple Cortex-A53 CPUs and how it compares with alternatives, because about a decade ago I have used many single-board computers with various kinds of such CPUs (including Raspberry Pi, but also other very different SoCs). I eventually abandoned them for better alternatives.

If you have modest requirements, you can be content with such a CPU, e.g. for reading and editing simple documents or browsing the Internet with scripting disabled.

The point is that there is no need to restrain yourself to cope with its limitations, because for more than a decade there have been much better alternatives.

Even when your target is a sub-$100 computer, it makes no sense to use any CPU weaker than a quadruple Cortex-A76, like in the many computer models using Rockchip CPUs or in the current Raspberry Pi. While there are much more sources of Cortex-A76 based computers, the number of those offering much faster Cortex-A78 based CPUs using Qualcomm or Mediatek SoCs, including this smartphone, are steadily increasing.