Origins of Theravada Buddhism

Namaste! Speaking of Theravada Buddhism, I've seen some authors conflating what we know now as Theravada with the whole Sthavira Nikaya / Sravakayana. So they would say that extinct Sravakayana schools like Sarvastivada - Vaibhasika, Sautrantika, Vatsiputriya - Pudgalavada are a part of the Theravada Buddhism itself. But I honestly doubt this is the case. I think what we call as Theravada Buddhism nowadays (or perhaps we can call it "Tamraparniya") is the only remaining sect of Vibhajjavada (which itself is one of many sects of Sthavira Nikaya / Sravakayana). Its doctrines was only codified by Buddhaghosa during 5th century CE. Even before 12th century, Buddhism in Sri Lanka wasn't even uniform. We had Mahavihara monastery where the orthodox Sravakayanins were centered, then we had Abhyagiri monastery where the Mahayanins were centered. If I remember correctly, it was during 12th century, a Sri Lankan king unified Buddhism in Sri Lanka with the Mahavihara monastery as the lead of Sri Lankan Buddhism, and then its teachings spread to the Southeast Asian nations (i.e Myanmar, Thailand,etc.). Would it be correct & safe to say that Theravada Buddhism (as we know today) actually originated from Anuradhapura Maha Viharaya in Sri Lanka? Thank you!