Why is OpenWRTInvasion showing up as a virus to me?
Although you haven't described how "virus" warning is being presented to you, you should ask the maintainers of OpenWRTInvasion. That is a fork that is not official OpenWrt, so the community here is not well equipped to answer questions about that version.
All clear. I have another question then. How do I install OpenWRT on my Mi Router 4C?
Technically OpenWrt Invasion is a malware, since it exploits stock Xiaomi firmware. It doesn't do anything except that though.