Technically true, but more accurately, they are basically a physical EQ filter, that (at least in perfect world) ducks down every frequency a similar amount of dB, to not alter the music too much except the level, so the music on stage still sounds the same, but quieter. You can find frequency graphs in the manual (disclaimer: I have bought the Alpine ones and they are good).

https://acscustom.com/uk/products/other-products/sleep-sound

But yeah even they say that for maximum noise cancelling you want foam. These are better for comfort.