This website is a throwback to a design choice that (thankfully) has mostly died off - choosing a mid-grey for your text, making it much more effort to read. I used to have Stylebot pinned to my extensions to fix it, but haven't had to do it in ages. Designers - please don't do this. (I think it comes from people designing on much higher contrast Apple monitors and not testing on anything else)
But you are right, the Tuta article has a contrast ratio of 4.35:1, which doesn't pass even the lower WCAG level (AA = 4.5:1).
Accessibility is important. We are all going to depend on it if we live long enough.