They do have a boot loader that speaks uart, but are also compatible with jtag and openocd. However, I haven’t needed to burn the pins on jtag because they’ve implemented a gdb stub over uart, including the ability to trap interrupts and dump into a paused gdb session. It’s not ideal, but it’s pretty impressive from a preexisting tools perspective.

This doesn’t really answer your question, but it feels like it’s worth mentioning.