Regardless, they could improve it at any time, yet its been like this for... a decade now? Cognizant Inaction Is Intention. It doesn't matter why it is how it is; they can make it whateverthehell they want, they run the code, they run the org, they got the keys to production, so: fix it!
The original color may not have been malicious, but it WAS DELIBERATE. Every design decision at Apple is deliberate; to assert otherwise is to be revisionist. Cognizant Inaction Is Intention. Its color was deliberately chosen originally; its color was deliberately changed; their inaction in fixing the accessibility issues that change created is Deliberate, because Cognizant Inaction is Intention.
[1] https://d2bs8hqp6qvsw6.cloudfront.net/article/images/750x750...
As others point out, there are accessibility options for people who want more contrast (and which benefit users with trouble reading these outbound messages in far more useful places).
He's pointing out that Apple considered green originally the best background color for texting, and then added their own protocol with a blue background.
So it's not like they deliberately relegated SMS to green because they were certain blue was better; if they believed blue was superior, they'd have used it from the beginning.