On devices that have some middleware (like kindles or pocketbooks) you can't assume than disabling a plugin that does a task will prevent the task from executing.
I have no developer experience with kindles but I'm going to assume that you cannot reproduce if you disable autosuspend and start koreader in "no framework mode".
In regular mode, where some kindle services are still running, my best bet would be to keep the autosuspend plugin enabled and tweak its settings.
I might be wrong about kindle specifics, so please correct me
. It is what happens on pocketbooks, where the system does some stuff and the autostandby plugin deals with that stuff differently. In that case disabling the plugin will only enforce the default behaviour, while tweaking its settings is the only way to make the software/firmware to behave differently.