You're being tricked into believing in a nonsense argument. He is attacking a widely accepted historical view of succession without offering an actual contradiction. What he is doing is ahistorical.Buddhist sources of fifth century call Kushan emperor Kanishka as devout Buddhist. Kanishka died around 150 AD so gap is just 3 centuries. Historians now by examining inscriptions, coins and other evidences conclude that all those Buddhist texts are wrong and Kanishka was no more a Buddhist than he was follower of Greek religion. This when there is gap of just 3 centuries.
This is how rigorous standards are maintained in history, 'traditions' and 'beliefs' must be respected normally and outside historical domain but it is job of historians to separate fact from tradition and fiction. In this case, gap is of around 16 centuries, right? No competent historian uses such things without direct attestation.
In other thread, I have shown how even contemporary sources can be deceptive, Xuanzang described Harsha as Buddhist due to confusion and may use same but even a careful reading of Chinese account proves that Harsha was a Shaiva that is Hindu.