It's a terrible deal because most of their games barely sell anything on Xbox, a platform that has educated its userbase into not buying games and just playing what you get on Game Pass. So unless Sega gets a Game Pass deal (which they got for the latest releases of P3P, P4G, P5R and P3R) for their titles, it is simply not worth it at all to release the more niche stuff, such as Vanillaware's games, on Xbox, for the same reasons the majority of AA and sometimes even AAA Japanese titles skip the Xbox platform.
In the case of Unicorn Overlord in particular, the opportunity cost of developing and publishing an Xbox version at launch is too high for the little to non-existent return they'll get from it. They'd be much better off targeting only Switch and PlayStation instead, and these news only reinforce this analysis.
Or why do you think, sticking with Sega, the mainline Yakuza or Persona entries never had contemplated Xbox releases up until Microsoft literally financed the development of such ports?