I believe the explanation is "it's hard, it's being worked on, but it will take some time until all the pieces are in place", and they're not going to hold off releases until it is.

This release changes the ngl renderer to be the default renderer.

The intent of this change is to get wider testing and verify that
the new renderers are production-ready. If significant problems
show up, we will revert this change for 4.14.

You can still override the renderer choice using the GSK_RENDERER
environment variable.

Since ngl can handle fractional scaling much better than the old gl
renderer, we allow fractional scaling by default with gl now. If you
are using the old gl renderer (e.g. because your system is limited to
GLES2), you can disable fractional scaling by setting the GDK_DEBUG
environment variable to include the gl-no-fractional key.

This is what I've found here

This submitted article is far from a comprehensive changelog, and kinda glosses over some stuff, as you say.