Wow! This is actually an amazing feature. Rendering a list of items in which the order can change has always been annoying. I can see this feature greatly improving the situation.
Flexbox allowed this within a flat list, but this is much more applicable
reordering elements with CSS does not reorder them in the accessibility tree and thus is not really a good option in my opinion.
I've actually abused this feature in the past to satisfy bone headed layout decisions from on high that would have a dramatic negative effect on accessibility