"Support" in that context means you can boot the board. There's a ton more code required to get a functional media device!
Overall I am very happy to see a mix of the Pi Foundation staff, several core subcontractors they hired, and numerous names from the Pi ecosystem (including some of our own contributors) writing and upstreaming code to the kernel right now. The previous Pi architecture ended up being largely "out of tree" because when the first Pi launched in 2012 the folks who wrote the code were (in their own words) pretty ignorant about kernel things and the business priority was on shipping a working product. Over time their upstream awareness grew but the success of the Pi meant the task had become exponentially larger and still being a charity with a small staff the priority remained on shipping a working product. RPi0/1/2/3 are now super stable because the codebase has been iterated continuously and attentively over a period of seven years. RPi4 is being treated very differently. The benefits of being and business need to be "upstream" are very clearly understood now, and to continue the stratoshpheric success of Pi hardware the new shiny needs to mature a tad quicker than seven years. Lessons have been learned. Fortunately the success of the Pi means they're somewhat better funded this time around, so they aren't being shy about spending on good resources to get things done.
So .. to end my ramble. In the last few days some of the GBM/V4L2 work reached the point where we can package a "working" system on Linux 5.4 and we'll bump to Linux 5.5 soon. Proper public testing is still some way off as the transition from MMAL to GBM/V4L2 decoding means "one step forwards and two steps backwards" in the user experience due to the amount of new code involved. We'd like things to be closer to parity before we invite the masses to bug hunt. The greatest compliment will be a bunch of users whining about updating and not noticing any difference 