The Greater Maghada thesis is one of many theories about the religious landscape in the Ganges plains at the time of the Buddha.
The situation is complicated by the fact that we are separated from that time period by 2500 years plus, and our primary sources for that period are collections of texts passed down orally and then set down to manuscripts. Through that process, they went through differing periods of editing and redactment. That’s quite a minefield to wade through for the modern scholar.
It is further complicated by the resurgence of hindutva nationalism in “academic” circles in India, as extremists attempt to lay a hindu garb on other Dharmic religions like Buddhism and Jainism. This has polluted academic discourse quite a bit.
I think both Bronkhorst and the OP video make some decent points, but both are incorrect on cerain points as they seem to be racing to the opposite extremes: the Sramanas were neither separate from Brahmanism, nor were they completely formed from it.
The Vedic religion came from the west, but as it moved eastwards toward the Ganges, it began sweeping up pre-Vedic traditions as well; by way of example, the devotionals to deities in obviously yogic poses from the Indus River Civilzation. Around the time of the Buddha, tge gangetic plain was undergoing massive urbanisation and culture changes, and us such the primarily village based, agricultural Vedic Brahmanism was no longer speaking to the new urban elites; some studies of the suttas and theri/theragathas show a significant proportion of the early Sangha were probably urban Brahmins. In addition, some of the teachings in the suttas seem to be counter arguments to stuff in the Upanishads.
So, as these new urbanites were looking for new spiritual pathways, sometimes in methods influenced by brahmanism, in other times in ways that reflect beliefs the pre-date the arrival of the vedas, but were still influenced by them in the breach, if nothing else.
I am not really sure that you can draw a nice little dividing line between brahmanism and the sramanas; I see them more or less as two vines that are constantly growing around, and in opposition to, each other. And now, after 25 centuries, I am unsure if we will ever be able to entirely separate them.