The Sony Xperia 1 II may be the consumer version of the Xperia PRO, but it still contains many of the latter's professional features. Its latest software update gives it one of the Xperia PRO's best features too, arguably. The feature will be available on the Xperia 5 II, also.
https://www.notebookcheck.net/The-Sony-Xperia-1-II-and-Xperia-5-II-have-gained-an-exciting-feature-in-their-latest-software-updates.519565.0.html
I think the camera is meant to be connected over USB, not HDMI as USB-C to HDMI adapers aren't unidirectional.
If you sell a 'niche' professional feature/product, don't expect 'consumer' levels of sales.
Also, I highly doubt professional photographers are going to get an Xperia phone so it can be used as a monitor during photoshoots. Your average smartphone consumers are also not going to buy a professional camera to use this feature.
It's just a gimmick to keep the few Sony fanboys and hardcore enthusiasts happy.