According to Surah al-Maidah (5:72) those who say Jesus is God are committing the act of kufr and shirk and Allah has made Jannah haram for them. This ayah is in the past tense. One can easily conclude that even those who were born before Mohammed (sws) and had such beliefs will never enter Jannah. This invalidates your understanding that it was only the rejecters from among the direct addressees of the Prophet (sws) who can be called kaafir.
If we study the verse immediately following this one we learn that these verses only refer to the direct addressees of the Prophet. There are many textual proofs, which do not permit us to generalize the ruling you have drawn from the verses. However, this principle cannot be challenged that the Almighty would not forgive the crime of shirk even to those who have never been approached by any divine Messenger or Books. But falling into committing shirk in some cases and adopting polytheism as one's religion are two different things. The difference is so pronounced that the Almighty even after the Prophet's itmaam e hujjah on the Jews and Christians of his time, and even God new and the Prophet (sws) was told by God that they were knowingly denying the truth, the Almighty never gave them the same punishment as He ordained for the polytheists of Arabia. (For detail please study Surah al-Tawbah 9)
Now I move to your specific question regarding the use of the past tense. Past tense in Arabic can be used to connote past, present or future tense. It can be used to connote mere intention to do or start of an activity.
It connotes permanence in some instances and in perfection in some others. Therefore it is the context that would determine the meanings. Besides, there is a very clear indication in the succeeding verse that would remove your question fully.
What follows is the translation of the verse 73 of the Surah.
Certainly they disbelieve who say: Surely Allah is the third (person) of the three; while there is no god but the one Allah, and if they desist not from what they say, a painful chastisement shall befall those among them who disbelieve.
I believe the underlined part of the translation of the verse explains your objection fully.