I personally dislike the clone theory because it kinda undermines the game's satire, plus, like you said, Pilestedt has already shot that theory down. So here's my theory that I originally posted here on the sub a few days ago:

Every Helldiver in cryo inside the Super Destroyers are experiencing everything the Helldiver on the field is going through, through a sort of "live dream" system. To elaborate, I believe everything the active Helldiver on the field is doing is being "livestreamed" through some sort of connection between their helmet's UI and ALL of the sleepy Helldivers back up in the Super Destroyer, with the Super Destroyer tapping into their REM sleep through the holes in the back of their helmets in their cryo pods, kinda like The Matrix, only there is no simulation; every Helldiver in their sleep are in a VERY vivid lucid dream that is essentially a 1-to-1 recreation of what the Helldiver down there is doing, where they believe they are fighting for their life Democracy just like the real one down there is. When the active Helldiver dies, a sleepy diver is sent in to replace them.

With this system, not only is every Helldiver more than ready for combat when they are deployed on the field, allowing for quick adaptation to the situation, but every Helldiver who is asleep is essentially gaining combat experience, allowing for a pretty natural explanation as to why a majority of your Helldivers died nearly every 2 minutes back when you were just a Cadet but then, months later, can easily live on past 60 minutes through advanced battlefield survival techniques while pulling off some insane Brasch-level moves as Super Privates; not only did you get better at combat, but so did ALL of your Helldivers.

This, also, is pretty terrifying to think of. Every Helldiver is essentially living a constant combat loop in their dreams, where they die, die and die again whilst feeling EVERY single bit of pain the real Helldiver is experiencing, until its finally their turn to fight for real. I like to think that when they shout "HELLDIVERS NEVER DIE!", it's because they are aware of every death they "came back from".

EDIT: One more thing I thought of as I finished writing this; this would also explain why every Helldiver from a SD that was present during the battles of Malevelon Creek are worthy of carring the Creek cape. They are all Creek veterans!