It is very backwards compatible. I'm not sure why you thought that.
We jumped through quite a lot of hoops to make sure old software will be able to display new images. They simply won't display them optimally. But for the most part, that would be because the old software wouldn't display images optimally anyway. So the limit was the software, not the format.
What I mean by this is old software that treats everything as sRGB wouldn't correctly show a Display P3 image anyway. But we made sure it will still display the image as correctly as it could.
The sample HDR images don't show correctly in image viewers even though the colors used fit into the sRGB gamut (or at least have good approximations in there). That's not really backwards compatibility.
We jumped through quite a lot of hoops to make sure old software will be able to display new images. They simply won't display them optimally. But for the most part, that would be because the old software wouldn't display images optimally anyway. So the limit was the software, not the format.
What I mean by this is old software that treats everything as sRGB wouldn't correctly show a Display P3 image anyway. But we made sure it will still display the image as correctly as it could.