PIO stands for Progammable I/O, and it is a peripheral that is part of the RP2040 SoC, which is much more flexible than hardware implementations of specific protocols like SPI, I2C, UART etc. It can implement all these protocols and more at high speed and on any GPIO pins.
It runs in up to 8 special processors, known as State Machines, which are programmed in assembler using a machine language designed specifically for fast cycle-accurate I/O. These processors run independently of the main CPUs.
This implementation has been done from the specification, without access to any Raspberry Pi HDL. It is currently incomplete, but some programs run in simulation and on open source FPGA boards.
The current method of configuring and controlling PIO from a top-level module is different from that used on the RP2040 chip, and will probably be changed for closer compatibility.
For use from a host processor, such as one running micropython, an SPI read/write memory interface could be added. This would be a lot slower than a bus interface but speed is not usually an issue for configuration and control. There are usually too few pins between a host processor and the fpga to implement a 32-bit (or even an 8-bit) bus interface.