One job I got into some fights with marketing and our "design experts" over this because they decided the marketing image of the company was "light and airy" and so they wanted an ultra light font weight everywhere. It was their branding font they wanted for both headers and body text. (The were so focused on ultra-light fonts, to the point where they were even trying to get rid of the option to use strong text in the middle of paragraphs. If Outlook/Word allowed a group policy to disable the Bold button, they'd probably have taken it away. Haha.)

It took a lot of convincing that a software developer would know basic typography enough to know it was a bad idea, much less was I able to convey that it was an accessibility nightmare. I had to use some CSS tricks to get that work done and psychologically "show" that things were still "light and airy" while using reasonable font-weights for body text, and in the end they still didn't entirely believe me about the accessibility issues.

Couldn't people just set their user agents to ignore or replace lightweight fonts with standard ones?

Yeah the color choices for the graph are questionable too. They definitely don't pass contrast tests.