They picked a tint of a color used on low priority text. Someone argued that this particular tint is slightly worse for certain people. If you don't have vision problems, it's not really worse for you at all. We're talking about small differences in relative contrast between different elements. If you do have vision problems, you can easily make it and other similar situations across the entire platform easier to read with an accessibility toggle.
No, I don't buy the argument that the UX for reading SMS messages is meaningfully worse than for iMessages.