This is the "authentic" way.
Let’s be transparent. While the mod content (the custom songs and charts) is original fan creation, the ISO wrapper contains copyrighted code by Harmonix and Activision, as well as copyrighted master recordings by various rock bands.
How to stay ethical:
You have two primary ways to shred this new ISO.
Reputable uploaders provide an MD5 checksum. Compare this to ensure the file hasn't been tampered with.
This is the easiest way to get "Extreme Vol 2" running in 4K.
Pro Tip: Enable "Manual Hardware Fixes" and set the Rendering Clamp to "Full" to prevent graphical glitches unique to this mod.