Even with a microkernel, there's a lot of power in having a text console even when half the system is not functioning. If this code exists in a separate process, a lot of the handling bugs still need to be fixed to make the OS usable.
okay... and something needs to be able to decode the text sent to it over the serial port. All the bugs pointed out in the article also happen if you use the serial port console
Yes, and that “something” can be an app like gnome-terminal running on a full-fat text rendering library and GUI framework… on a completely different computer. That software could hypothetically have the same bug, but probably not, and at any rate it‘s not taking up space in the core of a microkernel.
The 120k+ lines of code in drivers/tty/serial in Linux suggests that maybe it's not _that_ trivial.