For a handful of developers to just "run through the code" and fix everything that easily? And do it quickly, without introducing other bugs?
I am not a developer, but I can remember writing software whether for BASIC, Pascal or Perl and going back to fix or extend something and seeing stuff and saying "Why did I do it that way?" and making changes that I'm not honestly sure were "improvements" except for they seemed like improvements at the time even though they may have created new bugs.
I don't know anything about the internals of OpenSSL so maybe it is that easy, but it makes me wonder why it hasn't been done before. But I suspect it is complex and having a lot of people committing changes all at once seems like it runs the risk of working a cross-purposes without a lot of coordination (which, maybe they have).