That doesn't seem unusual to me, given that to get to this page you either have to be searching for the specific terms already (and know what they are) or come from the homepage -> RP2040 (Raspberry Pi Pico) projects -> Custom serial bootloader for the RP2040 -> Preliminary reading RP2040 boot sequence
I came to that page directly from the front page of HN. I think it's reasonable to assume a significant portion of their traffic today directly to this page didn't already know what RP2040 is. Missed opportunity to educate readers.
I mean, in as much as it is annoying when an article about, say, NTFS internals does not explain what NTFS stands for, or what a filesystem is. If you're the target audience, you'll know already.