I went with "none of them were," but I think there is definitely room for misunderstanding. I think I'd have to hear it in context and out loud. It's a poor way to phrase it either way. Really, it should be "not all of them were . . ." or "none of them were . . ."
I had a similar problem back in college with an ad that got posted on some of the tables in the dining hall. It was an anti-alcohol abuse campaign, and the slogan was "Everyone doesn't drink." It seems clear that they meant "Not everyone drinks," but it could also be interpreted as "No one drinks." I grimaced every time I saw that stupid ad.
<--- From California, central coast region