In RGB, any curve applied on all three channels also changes the R:G:B intensity ratios per pixel, by nature,
mainly effecting saturation and at times also the hue.
But the Curve in LR/ACR is a hue protected curve...meaning it only impacts luminance and saturation (2/3 of the HSL potential). A normal tone curve in Photoshop does twist the hue, saturation and luminance. That's where the differences are. And again, Thomas did this on purpose. In my experience, when Thomas decides to something a certain way, he's usually right.