I think you are confusing what is possible as far as construction timelines go with the always much slower Disney construction timelines.
Other parks can demolish existing rides and build massive coasters in 8 months. Take, for instance, Cedar Point. I was just there and one of my favorite coasters was Gatekeeper. It is a marvelous 170 foot tall, 4160 foot long B&M wing coaster. Cedar Point built it where 2 other rides were previously. It took them 8 months total to demolish the 2 existing rides (an indoor coaster and a 300+ foot tall sky tower), clear the land, and build this new coaster. That also included completely reconstructing their main entrance gate with 2 tall keyhole towers the coaster twists through (hence the name Gatekeeper). All that work started August 6, 2012. The coaster was completed and testing on April 4, 2013. It opened to the public on May 11, 2013.
By comparison the work to build the alien ride itself is way simpler and on a way smaller footprint. It is a simple flat ride with under-floor mechanics. If someone were to build it on a piece of flat land it would like take them 4 months or less to build the whole thing.