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The cones are the colour sensitive portion of the retina, but only make up a small percent of all the light detecting cells. The rods (more or less the brightness detecting cells) would still function in a deuteranopic person, so their luminance perception would basically be unaffected.

Also there’s something to be said about the fact that the eye is a squishy analog device, and so even if the medium wavelengths cones are deficient, long wavelength cones (red-ish) have overlap in their light sensitivities along with medium cones so…

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The rods are only active in low-light conditions; they're fully active under the moon and stars, or partially active under a dim street light. Under normal lighting conditions, every rod is fully saturated, so they make no contribution to vision. (Some recent papers have pushed back against this orthodox model of rods and cones, but it's good enough for practical use.)

This assumption that rods are "the luminance cells" is an easy mistake to make. It's particularly annoying that the rods have a sensitivity peak between the blue and green cones [1], so it feels like they should contribute to colour perception, but they just don't.

[1]: https://en.wikipedia.org/wiki/Rod_cell#/media/File:Cone-abso...

Consider myself educated, thanks!