There is a vast difference between a "source" and a "reliable source". Any place you find and use information is a "source". The website seems to contact some primary sources, which are usually the most reliable sources available, although what they seem to refer to as primary sources are books. Many would dispute that they are not primary, but typically secondary sources, which are a little less reliable. Secondary sources are those that are copied from another primary source, and as such can be prone to errors due to copying and/or interpretation. What they seem to regard as secondary sources are items that have the potential to support some of their primary sources from a historical perspective, but not so much from a genealogical standpoint.
So I would think you "can" use the website as a piece of the sources for profiles, but I would want to have other supporting sources to back up what I found there and not use them as my sole source. I think there's just enough room for error that I wouldn't expect that some of their information could be in error due to either misinterpretation, conflation, or plain old copying errors. I'd expect that most of it has been fact checked and cleared from a historical perspective, but I'd just be cautious regardless.