Pre-Sectarian Buddhism I believe there are alleged to be 24 different schools of early Buddhism, the only surviving one being Theravada, which preserved the Pali Canon in Sri Lanka. Buddhism in India mysteriously disappeared sometime during the Gupta Empire.
What your sources seem to suggest is that the Buddha, rather than being a historical figure later mythologized in Mahayana, might have originally been a mythological figure later historicized in Theravada. It's an interesting idea, but I want to say that this would be on the fringe of Buddhist scholarship, much like the view that Jesus wasn't a historical person. I happen to like the work of Robert M Price concerning the latter view but am personally unpersuaded as to its validity, just as I am unpersuaded as to the idea that Mahayana is more ancient than Theravada. The role of language is perhaps most important here, since Pali was a vernacular, non-priestly language of northern India (i.e. not Sanskrit, though somewhat related to it), which fits with the notion that Buddhism began in part as a reaction against elite Vedic religion.
Moreover, many of the concepts that became prominent in Mahayana are present in Theravada. Emptiness, the idea and role of a bodhisattva, Buddha nature, etc. It's just that these ideas get tweaked and transformed over time, often as a result of syncretic processes that occurred in the countries to which Buddhism spread outside of India.
I would also add a third school, Vajrayana (in Tibet and Mongolia), which sees itself as the third turning of the wheel of dharma, and so separate in some sense from the first two turnings (Theravada and Mahayana).
Lastly, I'm curious about you Wayfarer. I think you've said before that you identify as a Buddhist. In what sense do you do so? Have you formally taken the three refuges at a temple or monastery? And what form or school of Buddhism do you identify with? There aren't any Buddhist monasteries or meditation centers where I live currently, but even if there were, I would be hesitant to join one, seeing as Buddhism in the West and America seems to attract leftist hipster peaceniks who think Buddhism is just Marxism with oriental imagery. I suppose I would probably want to find something in the Theravada tradition.