It was only a matter of time before someone in the DIY mechanical keyboard community made the first mechanical keyboard with the Pico. I think people have been awaiting support from existing open source firmwares though.
There have been many keyboards PCBs designed for other microcontroller boards, such as Pro Micro, Teensy, Blue Pill and others. With a separate MCU board, you'd get an inexpensive keyboard with only through-hole components, that most builders could assemble themselves with only a soldering iron.
Having diodes in banks at the top is bit of a trend right now . It's so that there will be no exposed pad in-between the key switches on the PCB, as that portion does not have a covering plate.
It is common to cover the diode bank and MCU board with a piece of acrylic though, to protect against shorts.