The point here is that this surah indicates what other passages of the Quran also indicate: Allah sends messengers to people to command and forbid them, sending them as bearers of glad tidings and warnings. As He, the Exalted, says:

“We send the messengers only as bearers of glad tidings and warnings.”

They warn those who do evil of the punishments for their deeds, and give those who believe and do righteous deeds glad tidings of everlasting bliss and “that they will have a good reward,” “in which they will remain forever.”

Thus, His words,

“Those who disbelieved among the People of the Book and the polytheists were not to be left alone until the clear evidence came to them,”

explain that Allah would not leave the disbelievers as they were, in their disbelief. Rather, He does not leave them alone without first sending them the Messenger as a bearer of glad tidings and warnings,

“so that He may repay those who do evil for what they have done and reward those who do good with the best.”

This is further clarified by the fact that hattā (“until”) is a particle marking an endpoint, and what follows that endpoint differs from what precedes it, as in His words:

“until the white thread of dawn becomes distinct to you from the black thread,”

and His words,

“until they become pure,”

and His words,

“until she marries a husband other than him,”

and similar passages. If the intended meaning were that they would not desist from disbelief and believe until the truth became clear to them, it would follow that, after the clear evidence came, they all desisted and believed, since the wording encompasses them all.