“That is why the exception does not use the Greek word moixeía. It is intended for marriages in the betrothal period that would not go forward and be completed as “what God has joined together.” ”
Actually, no one can say why the Holy Spirit inspired one word or another. That includes you. So that is an invalid, speculative argument that must be rejected.
Nor does the context mention a betrothal period.
The context applied to how men were practicing divorce for various reasons, citing Moses’ words, during any time in a marriage.
Christ’s answer limited the reason for divorce to one, but put no limit on the timeframe.
More than that adds to the passage.