I think it is pretty obvious D/H would be higher for a comet that has been out a long time and that doesn't really tell us much about if Earth's water came from

I think it is pretty obvious D/H would be higher for a comet that has been out a long time and that doesn't really tell us much about if Earth's water came from comets a long time ago since those crashed early. H is a lot lighter than D, so it is a lot easier for a body to lose it, especially if the body has no magnetic field like a comet or Venus (D/H is also very high on Venus). Thinking the ratios would be the same seems like a mistake. Their graph should really compare the values normalized for loss rates and time since solar system formation.