Zoro is always in base lol. It's not like he has a transformation like Luffy. So what you see as Luffy's "strongest form" isn't much different as "Zoro's base". And in Luffy's strongest form, he has huge variety of attacks when in Zoro's case, his strongest attack is Asura
And yes Luffy used FS, but while doing that he is completely trolling an opponent superior to the one pushing Zoro back, showing the gap in their physical reaction / combat speed, and all that without showing higher level of armament / conqueror that Zoro possess, but still showing the big gap in power with a superior opponent to someone who Oda had to use an external plot device(Stussy) to let the Zoro vs Kaku inconclusive
Pretty much says everything