IOC, what with it being Initial, can be whatever you require it to be. The Marines had in mind what they wanted and achieved it, all be it clearly short of what Gilmore had in mind. Presumably the USMC F-35 OT team were working to HQ USMC requirements, not DoD OT&E requirements. Pentagon politics being what they are I am not in the slightest bit surprised by this.
Deployment on Ops early on is not necessarily a good way of doing things (F-111 example in Vietnam is a good example). Typhoon IOC was a long way short of what Typhoon was initially touted to be (Jag replacement etc) but it acheived a measure of operational success in Libya in 2011. F-22 was IOC long before it deployed on Ops in Syria (as far as I know), but then the nature of warfare recently has not called for high end A-A assets engaging in BVR missile shots and ACM.
The UK has taken the step of splitting IOC into 2, IOC from land bases is slated about a year before IOC from the Carriers for a very good reason. Walk, crawl, run etc. for the RN and RAF after the 'holiday' of Carrier Strike, and the need to work up a brand new ship as well as brand new aircraft. The USMC used a different approach, with well understood ship procedures (despite a majority of ship's company being changed between DT-1, DT-2 and OT-1) and just the challenges of integrating a single new aircraft type. These challenges should not be underestimated but you don't need a full ACE on board to test whether 6 F-35s are more of a burden than 6 AV-8Bs.