The general understanding was that "gender reassignment" in the EA was to cover people who had or who were planning to undergo surgery, or were planning to or had applied for a GRC. (Which in turn was generally assumed to apply to people who had or who were planning to undergo surgery.)
This obviously wasn't very well thought out, but it was thought of as a clear subset of people, with there being a clear process, much like a citizenship or residence application, not just a man saying "I'm a woman now".
Obviously many of those actually pushing for the laws were seeing them as a mechanism that could be twisted into men being able to just say "I'm a woman now", but that wasn't what lawmakers generally thought they were doing, and many were actively trying to make sure they didn't create that situation.