“they would not understand unless they were already keeping the feast.”
Non sequitur - I understand those terms and concepts but don’t keep the feast.
Only because you live in a society that has had the bible at the center of society for centuries. Western society in effect is (or at least was) built around it.
The gentiles of that time were not so. They worshipped completely differently and were for the most part wholly ignorant of the creator God. Paul makes that point at the Acropolis.
If Paul had not been teaching and instructing about God's holy days when he used the term "paschal" (passover) the gentile converts would have been like "What the heck does he mean by that?" Same thing with the term "heortazo" (to keep one of the feast days of the bible").
In short Paul was teaching them spiritually by using terms and concepts that it would have been impossible for them to understand unless they knew the context.