If that's true, then it actually is entirely in-spec. I had read several articles claiming it was running at 4266MHz, which would definitely have been extremely out of JEDEC spec, but not unusual for current-gen DDR4 XMP profiles.
Some higher-end RAM even gets above 5100MHz / 10200MT/s https://www.crucial.de/memory/ddr4/blm2k8g51c19u4b, so I had expected Apple to get somewhere around 7600-8400MT/s at 16-19-19-36 or better timings.