Yeah, they'll be full games but FFXIV expansions basically have a full game's worth of new content as well. With the groundwork now laid, they should be able to hit the ground running with Part 2 because they will very likely be reusing as much assets etc. from Part 1 as they can + they should be pretty experienced with the workflow & engine, instead of having to start from scratch again. They are (likely) building on top of what they've done with this first release and not re-inventing the wheel, that's what I meant with "more like FFXIV". Someone who has designed monsters can just keep designing monsters for a combat system that they are already familiar with. A lot of the work can just resume as if they were developing content for the first part.
I don't think the whole "don't expect development to be much faster" means that they'll take anywhere near 5-6 years for Part 2, just that even if they've now created a full game, an FFVIIR quality upgrade of something like Wutai will still take a lot of work to create. They can't develop much faster, necessarily, but they can get a sequel out faster because not all of Part 1's development has been as efficient as the development can be in full swing. They should be able to hit that full swing with Part 2 much faster (again, so long as part 2 isn't a total overhaul because of the likely console generation change)