Basically, Amazon started building a new engine and plugin architecture. After acquiring the CryEngine code, it was pretty heavily refactored to fit into the new LY design and plugin (gem) architecture.
Overtime they had slowly replaced gems that were originally of Cry design, with LY's origin design untill finally the last major piece of Cry was it's renderer.
With the release of O3DE, it finally includes their own new renderer Atom, finishing their transition off of CryTech developed system.
So, while it's strictly true that LY is a CryEngine fork. And I don't think CryEngine 5 code was ever imported. It's really for the most part irrelevant, as it exists as an almost entirely original piece of technology now that has almost entirely lost it's Cryengine heritage by now.