Downloaded installer from project main page shortcut and ran in Windows 7 machine without a 7-Zip install. Ran with default options, resulting in a download of i686-5.2.0-release-posix-dwarf-rt_v1-rev-.7z. Install failed at end of download with message "The file has been downloaded incorrectly".
Once I installed the 7-Zip utility the installer ran fine.
The installer does not use a third-party 7zip.
I regret to report that you telling me that the installer doesn't use a third party 7zip does nothing to change the observed problem - the installer initially failed (tried several times), tried again after installing 7-Zip and it worked ;)