Well I don't really think that is the case - I'll point out that Carsland started construction in July 2009, and opened June 2012 - which was 3 years from start to finish - and that would be a less ambitious project than this is, and that was built in a parking lot. I suppose that's still faster than much of the stuff at WDW, it's not THAT fast.
(Side note: I was trying to think of the last new attraction in Disneyland itself to open - not counting the subs which were a redo of an old attraction - is "Temple of the Forbidden Eye" really the last NEW ride opened at Disneyland? In 1995? 21 years ago? Geez, and we complain about WDW?)
My assumption was that the Railroad/Rivers of America is a fairly minor component of the overall construction job, and there will be done long before the actual construction - yet THAT job is expected to take 18 months. I just don't see Star Wars land from where it's at today being done in 18 months. Late 2018 would be the soonest, and I think 2019 is the reality.