It turns out that there's not been a release of AVRdude for a while, and all of the additions of support for UPDI have only been recently added. So you need to fetch the trunk tip of the repository (see the linked project) and build it for yourself. But having done that, you can, for example:

avrdude -c serialupdi -p t3226 -P /dev/ttyUSB0 -U flash:blinky.hex