Does the SIP still protect against other operating systems?
Solution 1:
No, System Integrity Protection is not active when you are running other operating systems or when you mount the hard drive in another computer.
There are other protection mechanisms (such as Secure Boot and T2 disk encryption) on modern Macs that protect your system from booting "malicious" (i.e. infected) operating systems, and from the disk being simply mounted on a different computer.