The webdesign.about.com page is simply wrong; they probably misunderstood the HTML5 drafts. Allowing DIV inside P would cause great confusion; I don’t think it has even ever been considered during HTML5 development.
If you try to use DIV inside P, the DIV start tag will implicitly close the P element. This probably explains the things you’ve seen.
To avoid the problem, do not use P if the content contains, or may contain, a DIV element. Use DIV instead.
