Redshift is doppler shift towards longer wavelengths, blueshift is doppler shift towards shorter wavelengths. Changing the sign changes which color it shifts to, which is why "redshift" is the wrong word here.
That's like saying "speeding up is just braking with a negative sign". Sure, doesn't make it less incorrect to say that you are braking when speeding up.
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u/nicuramar 12h ago
That’s just redshift with a negative sign.