I actually don't think the knowledge is as important as passion.  I'll try to keep this answer brief but I make no promises.

It is certainly important a for a community founder to have some knowledge, on the other hand, he or she is going to display it the manner that the members generally will... by posting reviews, comments, quick tips etc. in the community.  In short, it pretty much takes the effort of the founder and the members to demonstrate the knowledge of a community as a whole.  I say this because how much knowledge you have of a particular subject doesn't actually play a major role into creating the community.  And sometimes displaying too much knowledge is more alienating than it is inviting.

Let us say that a new member joins who doesn't have quite as much knowledge and the founder comes onto the page and instead of greeting him comments on the review going on and on and on and on about how the new member is wrong in his assertions or something along those lines.  You have no effectively discouraged a new member from wanting to be part of that community.  No one really likes it when someone gets on their high horse to demonstrate the vast amounts of knowledge they have.  On the internet especially you come off as arrogant more so than smart.  And in other cases you might come off as condescending. 

Likewise, being "extremely" knowledgeable in some communities is damn near impossible.  Let's take a look at the Cafe Libri community.  It encompasses ALL of literature into one community.  It's got thousands of members all bringing their own different opinions and observations to the table about tens of thousands of various books.  Consider just how broad the subject which Cafe Libri deals with is.  No matter how much knowledge ANY of Cafe Libri's members have about the subject of literature, there will be a lot MORE knowledge which they lack.  And this is primarily because Literature has such a broad history behind it as it is.  There are too many theories, philosophies etc. within literature and far too many different ideas.  And while knowledge OF those ideas can certainly help out... what good would demonstrating them to the wide range of members be?  Especially considering how many members there are. 

But just the same it's also about the application of that knowledge.  I know A LOT about video games.  I've been studying, researching and experimenting in the realm my entire life.  But a great deal of that knowledge won't help The Gaming Hub as a community.  Because much like Cafe Libri the subject is just too broad (not to mention an entirely different beast).  But I'm also willing to bet several of the members could contribute in ways I never actually thought possible. 

And let's be very honest here.  If you've got passion about something you've definitely got knowledge.  Sometimes more than you think.  But also consider that part of the reason the members who join the community do is because they might also share that same passion.  I guess my point is that we're looking at a situation where it takes a village.  It's important for the founder to have knowledge without a doubt, but if he lacks the ability to figure how much to apply (surprise it's not much) then it's not really that beneficial to the founder or the members of the community.