Red on white is almost indistinguishable to me from black on white. If you're trying to use red to indicate a state, it fails on me and I'm sure many others.
The parent's complaint is while the red may look black to you, you have no trouble differentiating it from the white background. The problem of red text on a white background would be if someone couldn't see red and had another impairment categorized as "low vision"; for them, red-on-white text the size found on HN would not be readable. If the red-on-white text was larger and bold (>18px, weight 700), its 3.99:1 ratio would be sufficient for WCAG's 3:1 large text threshold.