I guess the points being made on if the coaster will be built before the building or visa versa comes down to a couple of factors. The main one being the complexity of the track and the associated crane lifts required to get that into place. Simple structures such as the Thirteen drop structure were fairly basic so could easily be slid into place almost and hidden.
Full blown coasters such as Alton's Black Hole, The Tron and the Rock 'n' Rollercoasters all required pretty much full track construction before any building was put up around them, I suspect, mainly to do with how they need to crane and lift the track and supports into place.
Given that we do not know much about Horizon yet, it is entirely plausible that either option could be used. They may even try and push for an internal assembly if it is possible to keep it hidden.
I am sure Alton Towers and the construction companies know what method they will be using by now, just not clear to us at this point.
Alton's black hole could probably have been done inside the building, without a doubt very easily in this day an age. Over 30 years ago though things were a little more primitive meaning it would have been much harder and expensive to do it indoors.
For the Disney rides, given the side and complexity of the track and support structure, it would make it very difficult and expensive to do that indoors. You would need to install some sort of gantry crane into the roof. Expensive and pointless seeing as it will only be used once, easer to just use external tower cranes.