I haven't read the code, but I'm guessing the API they're using would allow them to track _all_ changes. The only reason Reddit works that way is because they choose to only keep one version of the comment internally (or at least they did back when most reddit deleters were in use, in 2017 or so). It's architecture, not some fundamental limitation, that causes that to work, although it may not actually work anymore. Only reddit knows that.