For what it's worth, someone posted this on ARRSE:
"Work mate of mine is ex sea harrier maintainer and still has a lot of contacts on the F35 circuit, apparently there are inner and outer intake blanks and its common for the blank to be used as a knee pad while inspecting the first stage compressor disc."
I couldn't see how a properly fitted intake blank could be missed by the aircraft start groundcrew, pilot on walk round, flight deck marshallers, Flight Deck Officer and those in FLYCO.
However, if the malpractice described has been going on, I can see how a folded blank laying in the intake might get missed, particularly if at night, and then not get sucked in until powered right up just prior to the take off roll. I'm still surprised anything laying folded in the intake wasn't sucked in immediately on start.