Two points.
First off, the Gospels were written AFTER Jesus had dies and rose again. People knew by then whether or not there were any more.
Second, the Holy Spirit is the Author of Scripture and I daresay HE already knew whether or not there would be more before the fact.
So if the Holy Spirit says “ firstborn” and not “only” the Jesus was the FIRSTborn. There were more that followed.
So if the Holy Spirit says firstborn and not only the Jesus was the FIRSTborn. There were more that followed.