Not sure how all NC earpones work, but I'd say you still need the delay to properly process the sound and mix it with whatever is playing (unless you rawdog the input flipped and amplified directly in analog).

Normally sounds don't change in frequency that often so that's good enough (tm). I can hear myself typing now (short burst of sound), but the washing machine nearby, which is louder without earphones, is completely gone.

Sorry, you’re right. My understanding was that it was the same in practical terms to delay by half a wavelength, but that wasn’t correct.