You could be right about the English definition of fornication & adultery. I was looking at it from the lens of the Adultery vs Fornication debate of Matthew 19:9 where the terms are more tightly defined.
But the NT was written in Greek so let's bring back the discussion to that. Porneia can be an umbrella term to include all kinds of sexual immorality but it can take on a specific meaning depending on the context that it's used as explained in other threads. This interpretation of porneia to include adultery in Matthew 19:9 is popular among evangelicals but it originated from a Catholic priest and a Christian humanist by the name of Erasmus in the 16th century.
Prior to that, the early church always interpreted the exception to be relevant to the bethrothal period of a Jewish wedding. This explanation would be consistent with Jesus not giving an exception for remarriage in Luke and Mark which were written to Gentiles.
Luke 16:18
Anyone who divorces his wife and marries another woman commits adultery, and the man who marries a divorced woman commits adultery.
Mark 10:11-12
He answered, “Anyone who divorces his wife and marries another woman commits adultery against her. And if she divorces her husband and marries another man, she commits adultery.”
Matthew also appeared after Luke and Mark so there was a period of time in the early church where the exception was not known.
https://www.danielrjennings.org/This...Remarriage.pdf