What is meant by the people being "a single ummah", and when was this?

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

The term "ummah" here refers to a group united by a single cause. Regarding the cause that they were initially upon, there are two opinions:

1. It was Islam. [Ubayy ibn Kaʿb, Qatādah, al-Suddī, Muqātil]
2. It was disbelief. [Reported by ʿAṭiyyah from Ibn ʿAbbās]

Regarding when "they" were a single community, there are five opinions:

1. It was when they were presented to Adam and acknowledged servitude to Allah. [Ubayy ibn Kaʿb]
2. It was during the time of Ibrāhīm, when they were disbelievers. [Ibn ʿAbbās]
3. It was between the time of Adam and Nūḥ. [Qatādah]
4. It was when they boarded the ark, as they were upon the truth. [Muqātil]
5. It was during the time of Adam. [Mentioned by Ibn al-Anbārī]