***** NOT A TEACHER *****
I do not know where you read that, but I most respectfully disagree.
Here are some sentences from a very good grammar book:
The ring is of gold.
The teacher is in a very bad humor.
He is out of funds.
The book says that those prepositional phrases are subjective complements.
IF you accept that opinion, then your sentence is simply a case of SVC.
Source: House and Harman, Descriptive English Grammar.
Student or Learner