Why would he assume that?
The FEC and the Senate Ethics Committee are two separate and distinct bodies, and the paperwork which makes one happy won't necessarily work for the other one. Each has their own set of rules to enforce, and their own distinct forms to be filled out.
Cruz is no country bumpkin trying to do a "Mr. Smith Goes To Washington" thing, he was already a well-connected Washington insider and a top-level lawyer, so his memory lapse claim doesn't ring true.
Nobody, but nobody was trying to hide anything.