What is the meaning of ẓann intended in the passage "Those who reckoned (yaẓunnun) they would meet God said"? And who is this description referring to?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. It means certainty. [Al-Suddī and others]
2. It means uncertain assumption. The intended meaning is that the party, due to their small numbers, assumed they would be killed and meet Allah. [Al-Zajjāj and others]

There are two opinions regarding who is described as "reckoning that they would meet God";

1. They were the 313 men (who drank no more than a scoop). They said to those turning back, "How often a small group has overcome a large group!" [Al-Suddī]
2. They were the most resolute and virtuous out of those three hundred and thirteen.