That's not the main reason. The main reason is perf/W for highly parallel workloads.
> The downclocking for AVX started with Knights Landing.
You're mixing things up here and that statement is incorrect too. AVX throttling started with Haswell-EP [1,3] (Intel kept it quite hush-hush avoiding mentioning it in products specs and such). Secondly, Xeon Phi is the HPC product family and KNL is the codename of the 2nd generation of these arch [2]
> My Boardwell-EP Xeon stays at 3.0Ghz even when I (ab)use AVX2.
In that case you're most likely either not using more than 1-2 cores or you're overclocking (or perhaps monitoring incorrectly), see [1,3].
[1] http://images.anandtech.com/doci/8423/AVXTurboHaswEP.png [2] https://en.wikipedia.org/wiki/Xeon_Phi [3] https://www.microway.com/knowledge-center-articles/detailed-...