I would have imagined someone in technology (or at least enough to read HN) would understand the hidden complexities of things they're not intimately familiar with
Pasteurization via heating the entire machine to a high temp and holding for some time is likely the simplest you can get. No need for cleaning solutions, flushing, etc. Simply heat the thing up for a few hours.
As for duration, that's a function of temperature and desired safety margin. The higher temp you go to sanitize, the faster you can sanitize, but if they're heating the ice cream mix as well, there's likely a limit to how high they can get without damaging/altering the mix. So the other variable is time, and that's probably what leads to the multi-hour sanitize cycle.
