The installer.php script does absolutely nothing more than fetch the indicated MODX zip file, unzip it, and redirect the browser to the MODX setup/install directory. Any leftover _build files or overwritten files are due to the MODX installer/setup, and will do exactly the same thing if you upload and extract the MODX .zip file yourself.
The main flaw with this installer.php script is that it doesn't do anything about a failed download. There is the try/catch in the download function, but nothing is actually done with it.