subordinate clause: as if they had been total strangers.
clause type: adverb clause
It’s a good rule of thumb to keep an eye out for subordinating conjunctions when looking for subordinating clauses because of how subordinating conjunctions (such as “as”) will begin a subordinate clause. Then, when trying to decide whether the clause is an adverb clause or an adjective clause, a good question to ask yourself is “What is this describing?” For instance, what is “as if they had been total strangers” describing? The answer is the way they boarded. And, “boarded” is an verb; thus, because the clause is adding to the verb, the clause is an adverb clause.