Despite it is a db9 connector, BK decided to have a TTL UART interface (*not* RS232) => no need of level shifters nor to invert the signals.
Huh. Disregard my comment then! I should have RTFM.
You know, I spent 30 seconds staring at those pictures wondering just
why it worked, and it never occurred to me that someone would actually put a TTL UART on a DB9.
I hope they have good protection on those pins.