LIGO Curvature Compensation

One argument in favor of curvature is that that when building the Laser Interferometer Gravitational-Wave Observatory (LIGO) its builders had to compensate for the curvature of the earth. On the LIGO website we read:

https://www.ligo.caltech.edu/page/facts (Archive)

Looking further at the matter, at the bottom of p.168 of Einstein’s Unfinished Symphony (Archive) we read additional details of LIGO's curvature compensation method. We see:

A Flat Earth Interpretation
The above says that the ends of each arms are situated several feet off the ground than the center station to "account for the curvature of the earth." Would it work if the earth was flat?

Yes, just as it is possible to see an object which is elevated several feet above your eye level 4 km in the distance, it is also possible to point a laser slightly downwards or upwards to hit it.

LIGO diagram:



The people at LIGO had to take steps to aim and align the components at some point. To hit the opposite marker they would have aimed the laser beam at the destination, or would have aligned the intermediate components so that the laser beam would hit the destination. The set up does not appear to be a physical obstacle on a FE.