Z13 owner here, not X13. But I think it's the same thing.
There are 'two modes' of HDR support. One for 'playing back HDR media through supported device', and one for actually producing HDR content in things like games.
Although Asus says 'HDR! Dolby Vision' that actually only means 'media playback HDR', and that is enabled in my HDR page. Which means apps like Netflix or others detect that you have HDR support and they playback the content like that. But gaming-HDR is not a thing on it, as far as I can see.
It's just a bit of marketing gimmicky... basically the same as saying a $500 TV has HDR, while it can't produce anything that resembles good HDR conditions anyway.
This is a checkbox so they can say 'HDR', but it doesn't actually mean 'gaming in HDR' :(.
Had a few more thoughts about this computer and this seemed like as good a place as any to post them...
I ended up returning the X13. It was for my better half, and there were just too many issues. Besides not actually being HDR, it wouldn't wake from sleep most of the time and it would only connect to a docking station about one out of every ten attempts (tried 4 different docking stations, same result). Her frustration becomes my frustration so the X13 had to go.
The main reason that I'm posting here though, is because I picked up a Flow X16 for myself. It's been 100% flawless so far and guess what? HDR works out of the box. Windows supports it. YouTube plays HDR. It's beautiful. I haven't even opened the stupid Dolby Vision app.
So what happened with the X13? Only Asus knows. I have a hard time believing that the same company made these two laptops let alone that they're supposed to be in the same product family...
Thank you so much for the explanation! Was struggling with the same question and my Flow x 16. Now everything is in its place. Netflix using Dolby vision plays HDR correctly. Bottom line - the whole mess will be over when Microsoft adds Dolby Vision support to Windows.
One of the few times a support person laid everything out in 1 post and closed the issue with that xD