Them not agreeing believers are saints is a guarantee.
Otherwise, they would not be disbelievers. The objection is more "if Mohammad (s) is true and his God exists then why not... this and that".
That doesn't make sense to be about believers being good or bad.
Those were the Ulama of Jews and Christians mostly, who considered themselves having the truth on their side, without considering the possibility that they could be wrong. They did not consider themselves in need of guidance this was their arrogance, as they considered themselves more knowledgeable than Muhammad, thus, in their mind, they knew Muhammad is a false prophet.
Thus they were making excuses and ridiculed Muhammad to reject Him, saying if Muhammad is a true Messenger of God, why He eats and walks like ordinary human beings. Why there no Angels with Him.
But consider, for example neither Jesus nor Moses were accompanied with Angels. So, how reasonable were they when they were saying, why no Angels are with Him?!
And how would that change on the day of judgement per your interpretation?
That is a very good question.
Let's read a bit farther from Surrah of Al-Furghan:
"Then the Messenger will say: "O my Lord! Truly my people took this Qur'an for just foolish nonsense." 25:30
This verse, is an allusion to the next Messenger after Muhammad. When Qaim comes to His people (Muslims), He says verse 25:30.
Now, lets look at next verse:
"And thus We have made for every Prophet an enemy, among the criminals. But sufficient is your Lord, (as) a Guide and a Helper." 25:31
This verse alone, is so worthy to ponder. It reveals mysteries of God!
Notice, it says, God is the one who makes enemies for every Prophet! But how, and why would God makes enemies for His Prophets?
Here I only focus on "How God makes enemies for Prophets, and why this verse is immediately after the verse about the Qaim"
Notice, this verse and the verse before it, are allusions, that, when Qaim comes to His people, who are Muslims, it is the Muslims who Allah made them enemies of the Qaim!! They are the ones who in reality are misguided!
This shouldn't be strange considering that the enemies of Jesus, were the Jews, the very same people who considered themselves as friends of Messiah!
But how does God prepares enemies for His Prophet and on this case the Qaim?
He does that, by misreading them, using Mutishabihat, and Parables in the Quran. Thus, this is the reason why the Quran has a part, which is Mutishabihat.
Now going back to the Term "Angels". By this term is mean, those firm Believers who Allah chooses as helpers of His Faith. But, God did not want to explicitly say the meaning of the Angel. This is just one example of how God uses Metaphors which is Mutishabihat. It causes many to be mislead. Through this misleading, they have false expectations from the Prophet. Thus, once the Prophet comes, they think He is a false prophet, and become His enemies.