As someone with strong deuteranopia (I struggle to differentiate shades of green and darker bluey-reds), I am extremely sceptical about that claim too. For what it's worth, I've never had a problem reading the white-on-green bubbles in Messages. I do agree that a contrast closer to WCAG's recommendation would be better (currently 2:1, recommendation is ~4.5:1), but this is a diversion. The point I was making is that no matter what Apple does here, there is visible differentiation, and people will complain about it.