No. "Assume it's copyrightable. It's still fair use." That means you don't have to answer "Is it copyrightable?" The Supreme Court generally prefers to not answer questions that they don't have to answer.
if (!isFairUse(workUnderInvestigation) && copyrightable(originalWork)) {
bigCopyrightPayout();
}
Short circuit on the and operator. It's fair use, so copyrightable will not be evaluated. Evaluating copyrightable has an obervable side effect of creating a precedent.