Pretty sure there is no text on the site which claims the sanity score to be a "valid scientific test". Though, if you knew much about psychology, you would understand that there are actually many rating scales that are considered legitimate which use self- rating and are also considered subjective.

I appreciate that you challenge the validity of the test but much of the value of the test actually lays in the repeated self-assessment by the same individual, in my opinion. While it may or may not be valid, if it is taken over time by the same person it is likely fairly reliable.