This chapter is about the Cosmological argument for God's existence, not the Ontological. Of course, Ontological arguments get their name from the term 'ontos' meaning existence, yet it is missleading and potentially detrimental that this is written as though it is about the ontological argument. I've not read the whole thing, though at a glance it is quite obviously mistaken. In short, whomever wrote this cannot have read the 11 paragraphs of Section IX in the DNR.
In truth, the first part of Demea's argument runs closely with Dr Samuel Clarke's cosmological argument. Clarke, however, goes into more particular reasoning as to why the first cause must be God (beyond the fact that it cannot be Chance or Nothing).