Turns out the property element wasn't actually necessary. At some point I thought that maybe I can submit the app without it and give the reason for the permission in some submittal form. It makes sense, if you think about it, what if I need to modify my explanation to correct or clarify something ? If it's in the manifest, I would need to rebuild the app and theoretically test it again completely. Which is nonsense.
Bottom line, it worked, and the app was even approved.

So I'm guessing the document I quoted initially, and a few more on the subject, are probably obsolete. I mentioned this in my bug report case at Google and asked them to check, if they can, no response so far.

In any case looks to me like this is clearly the best way to do it. As long as it stays like this.