ECC RAM overclocks worse than non-ECC.

Yes.

ECC modules are binned for stability and error correction, not speed. The extra chip adds complexity to the memory bus, and manufacturers tend to run more conservative timings. Even with the same ICs, the ECC version will have less headroom.

I’ve tried pushing a few pairs of ECC DDR4 sticks. On average, you’ll hit the wall 100–200 MHz sooner than with the non-ECC equivalent. Sometimes more if the ICs themselves are lower-binned (common for server-grade stuff). It’s just not what they’re made for.

If you need ECC for a workstation or server, leave the RAM at stock. Overclocking ECC is a gamble that usually doesn’t pay off.