This is a clickbait. On my iPhone the green color is much darker than what the author shows in the article. Here are some images with the hex color codes for example:
It's actually really cherry-picked samples. All messages (blue and green) in the Messages app fade out slightly as you scroll. The author picked a blue message from the bottom of the screen and a green one from the top. If only they scrolled around a bit... they would have noticed.
Great point. I just now confirmed that as well. The bottom message is much darker than the one on top
Also the article doesn't mention that the iMessage blue also fails the same WCAG test. I just ran it myself.
As shown in the image, the blue at least passes the test for "large text"
A comparison between blue and green bubbles on your phone would be more relevant. Color space handling makes direct comparisons between screen grabs (from different devices) tricky.
The author used the screen grabs from different places too. On iPhone you can't have the blue and green bubbles side by side
A reasonable start would be screen grabs of both blue and green bubbles, grabbed from the same phone in the same way. (And vertical placement; see the other comment.)
Here are the results for your image. Contrast still fails on WCAG guidelines.