That's strange to hear, in my experience serif fonts are most definitely used more for body text (referring to paragraphs of text) on sites where legibility is a primary concern. Look at the websites of almost any well designed news outlet (Bloomberg, NYT, Guardian, Reuters, etc). I say this as someone who previously worked for a type foundry that has worked with major organisations on corporate type projects and now works as a web (and occasional book) designer/developer.