> Two hours, for a single body, in a facility the size of a large garage. Just do some napkin math

I think you have mis-extrapolated the logistics.

It takes two hours for a single body, in a single-body crematorium, where the remains will be handled according to protocols developed for legal, social, and sanitary reasons.

In a truck-sized, industrial-efficiency waste disposal incinerator, with military protocols and no laws, I expect you could handle a much higher input rate.