[Question] HCT tone: why not use CAM16 "J" lightness?

[Artoria2e5]

The Science of Color & Design explains why HCT uses CAM16 Ch instead of CIELAB chroma:

You may wonder why we didn’t just use L*a*b*’s hue and chroma measures, then, we could have just use L*a*b*! However, when we tried using it in design, L*a*b* was too inconsistent perceptually.

But the article does not explain why HCT doesn't use CAM16 J for the "tone" value instead of CIELAB L*. Wouldn't we have the same benefit of only working with one standard color system and not having to glue together L* and Ch in the solver?

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Yeah, this is to get a quotable response to add to Wikipedia. But I'm also actually curious: is being able to get the contrast from the XYZ (skipping the CAM) that useful? Maybe there's some reasoning I've missed here?