Looks like they're using fat margins instead of max-width and nobody thought to use media queries to reduce them on mobile...

Edit: I should add the page renders fine on my phone, FF android. I'm not sure what's going on in the screenshot, maybe desktop view?

When done properly it is indeed a great practice.

But so many don't. The way the modern tech industry functions makes it hard to consistently get right, sadly. I've left two companies in the past two years where computational resources for automated testing and validation were simply not available. One didn't have any manual QA beyond the implementing developer. I'm in a better situation now, but those companies still exist. They're not exactly tiny startups either.

For all its strengths, without any amount of validation, RWD is very likely to lead to such issues. Unfortunately many industry execs are unsympathetic, seeing RWD as little more than a way to get two things for the price of one.