is F5 certified for Skype for Business?

I don't think so. It's certified (deployment guide + iApp) for Lync 2010 and 2013. So not yet for Skype for Business.

We are in the process of certifying for Skype for Business. I have successfully tested basic functionality using the current Lync iApp, which is also being used by the vendors doing the certification. Most likely the Skype for Business iApp will be a rebranded version of Lync, plus a few minor changes.

Mike