KEF's META is very interesting.
This video helped clarify for me they are talking about the tweeter's backwave, https://youtu.be/O_US3Kml0Cg
. Steve Guttenberg Audiophiliac rave review repeatedly saying the LS50 Meta sounds like an open baffle speaker https://youtu.be/oKTYvP7YQ0Q
left me with the impression the Meta structure was acting on both woofer and tweeter. This interior view shows the new cross brace that stiffens the box which in my experience helps woofer+box systems sound less 'boxie'.
KEF's talking points repeat that the meta absorber on the tweeter is one of several improvements that give the reported improvement in sound as this graphic details. BTW in this exploded view the META absorber is the three parts on the far right. These are assembled on the back of the driver with the tweeter's backwave delivered thru the center hole.
Searching "sub-wavelength resonators" starts a deep
dive into the topic like this article https://www.nature.com/articles/srep19519