Smerel, i made a similar mistake in taking the Tanya’s useage of that pasuk and thinking it’s in gemara – the gemaras maskanah is that it means cleansing, but i did think of a way to make the tanyas pshat fit in with a maharsha there, but it’s not the pashut pshat when you see the gemara.
He’s getting it from a zohar or arizal, kol tivu de’avdin etc…
either of which can take the pasuk to mean that al derech sod, but in pshat, that’s not how chazal understand it.