I agree with this to an extent...but you can make the same argument for an independent judiciary, for an independent press, local control of police, and so on.

Totalitarianism is resisted by a robust separation of powers. The totalitarian project is to corrupt every significant source of power and gather all to the leader (which we call the "state").

Leaders have, many times throughout history, usurped the power of religion. Indeed, the authoritarian has often come through religion--think Khamenei in Iran (totalitarianism was there for the taking, but he chose instead to divest some political power as a way to make his position more secure: the protests are not against him). Religion has no particular power to resist authoritarianism (indeed, modern-day moves towards authoritarianism often start with an appeal to religion--Ergodan in Turkey, Orban in Hungary, even Modi in India).

So I just don't see that secularism actually matters that much--it is more coincidental than causal that a number of totalitarians rejected religion (Stalin, Mao) because that was the thing to do at that point in history; Mussolini, for instance, was quite cozy with the church (not that he believed any of it, I imagine, as he was a declared atheist when young: it was all about power...but the point was that Italy wasn't that secular of a society, and the church was quickly bent to Mussolini's will).

If you are in a secular society, then the authoritarian is not required to arise through the religious branch on their path to totalitarianism (though even in a secular society, it doesn't hurt), so I guess by admitting extra paths to totalitarianism, secularism removes a bit of protection. But, conversely, secular societies tend to do better at establishing other sources of power (judiciary, etc.), so I'm not sure whether it isn't a wash overall.

In any case, any sort of idea that says "totalitarianism is because of secularism" seems badly overstated even if there is an element of truth to it.