The fact is Open Source is more secure by its nature and it's even true when there's exactly the same number of security flaws found in Open Source and closed source project. The reason of this is everyone can check if there are security flaws in Open Source projects, so nobody can hide anything (but some smart guys can keep them in secret till someone else discovers the flaw) and in closed source world just very limited number of people can check the code - so the chance to discover the flaw is lower. To sum this up:
Just because a bug was found in a closed source program doesn't prove anything. Lots of bugs are found in open projects too. In the Linux kernel, problems are very often not disclosed at all until the fix is in place. There's a whole business right now around keeping Linux bugs secret up until the patches are developed and go live.