There was plenty of "homebrew" stuff way back in the day that used Katana. This is by no means a first in that regard. Most use of Katana for homebrew died out around the time KOS replaced libdream as the legal homebrew SDK of choice.
That said, most homebrew sites back in the day didn't want anything to do with Katana/WinCE based homebrew. It is illegal to distribute without a license for the SDK (which Sega is of course not granting to anyone), plain and simple. Sega largely left us homebrewers alone because we didn't use their tools illegally.
Honestly, there's little to no reason to actually use Katana at this point. KOS is more developer friendly and supports network capabilities in a sensible manner (including the modem). Not to mention that it's actually legal to use. Sega would likely come down on someone if they tried to sell something using Katana -- it's possible that something that's freely distributed wouldn't incur Sega's wrath, but I wouldn't see any good reason to risk it anyway. KOS is free, legal to use, and has a community of people to actively support it.
Now that I've got that off my chest... I do think it'd be neat to see some real online homebrew for the Dreamcast. I spent an inordinate amount of time on KOS' network stack, so I'm actually somewhat disappointed that nobody's (publicly) made good use of it as of yet...