Wikipedia defines subject complement as being "a predicative expression that follows a linking verb (copula) and that complements the subject of the sentence by either (1) renaming it or (2) describing it."
Basically, giving detail to the subject.
First is not giving detail to the subject, it is giving detail to the verb.
Here is an example of a subject compliment:
"He was the first to win the competition."
If you mean "he placed first in the competition (and therefore won it)," we don't say it this way. Your sentence means that he won the competition prior in time to anybody else (other people won it later).