It's still a possibility that it's real. Consider the likelihood of this hypothetical sequence of events:
- When the mole inside FDev got hold of the roadmap and decided to leak it, the Fleet Carriers concept was still uncertain. It was still "reprioritised" and "delayed", and not allocated to particular update or on the update schedule at all. More work needed doing on them, so they were taken off the schedule temporarily.
- The community already had some information about the carriers - namely the old info about them being a Squadrons-only feature, but even FDev themselves didn't know what they wanted to do with them fully at the time.
- The mole knew about the Arx update and some of the minor updates, but didn't consider the detail important enough to pass on, OR did pass it on to the leaker, but the leaker himself didn't consider it important enough and decided to word it essentially as "don't expect much in between now and 2020, priority is only to keep Elite making money".
- So by the time of the leak, Fleet Carriers were totally up in the air, with no concrete information to leak, and the minor updates looked uninteresting and/or unimportant.
- Some time after the leak happened, FDev made its mind up about the Fleet Carriers, changing them to being personal to CMDRs, rather than Squadron specific. Then sometime later, decided they could complete them for the December minor update, and so stuck them back on the update schedule.
- Since the leaks stopped at two distinct events, we can surmise that the mole probably got caught out, or decided her job was at risk and went dark, meaning that the later-added information never got leaked.
I imagine a lot of things to do with Elite's development are fluid and open to change right up until a few months before release, and even then some things can't be set in stone until after the content comes out, what gauging player reactions and minor balancing etc... So it's feasible to me that the information that we now have about Fleet Carriers just wasn't available to the mole or the leaker at the time. And that doesn't mean that the leak is false by any means.
How to reconcile this above with spacelegs, FPS combat and base building? Well, a whole chunk of the team got moved over to work specifically on the 2020 update, so it might have been easy enough to have a wander over to where they're working and see that, vaguely, they're working on things like walking physics, gunplay, character models and base components, without having to see an update schedule at all.