It's using some sort of a custom installer that also downloads Cloudflare's BoringTun (https://github.com/cloudflare/boringtun) directly from the author's website (nyr[.]be), since Cloudflare doesn't seem to offer it as a binary release. Example:

    { wget -qO- https://wg.nyr[.]be/1/latest/download 2>/dev/null || curl -sL https://wg.nyr.be/1/latest/download ; } | tar xz -C /usr/local/sbin/ --wildcards 'boringtun-*/boringtun' --strip-components 1

I don't have time to go through it in more details, but I would definitely recommend an alternative way of installing it.