Ridiculous? This is pretty common issue for anyone who travels to US. Visa may be denied for whatever reason and tough luck on appeal. I am EU citizen and had similar experience just for visiting Iran on tourist trip. Do not even ask about guys from India, Pakistan or less fortunate countries.
And it got even worse with pandemic. US required vaccination for very long time, long long after it was relevant. Maybe they still do, frankly I do not care to look at this point!
I think biggest WTF here is why international organization like Mozilla is organizing company wide meetup in US, and not in country with liberal visa entry policy such as Mexico!