What do they mean by criticising the faith (īmān) of those they have described as "fools"?

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

1. They were referring to the religion of Islām. [Ibn ʿAbbās, al-Suddī]
2. They were referring to belief in the resurrection and recompense. [Mujāhid]
3. They criticized the open enmity between the two groups without consideration of consequences.

The first interpretation is based on the critics being Jews. The second and third interpretation are based on the critics being hypocrites.