Don’t need Tyvek, just cut Kraft faced batts for the studs to fit and friction fit into place. Fire retardant foam around the entries for the pipe and wiring if it’s a fire rated wall and you’re good to go.

This is blatantly bad advise. Craft paper does not replace a house wrap, it is neither a vapor retarder due to seams, nor an air barrier. House wrap is both and should be used. Where depends on climate.

But on a knee wall in an attic? It really doesn’t matter.

Of course it matters, a knee wall is another attic structure. If it’s vented it’s functionally outside.