Depends on the model of car, but most vehicles expose all of there memories (even runtime RAM) via a special per manufacturer mode, Now I have 0 doubt the devices out there cannot possibly do what they are claiming, though they can kinda do something, even if its the wrong thing to do, Reset the ECU memory every time it starts up the the stored 100Km economy average is above what feels "normal" In my own experience this tends to make the fuel range and average economy values on most instrument clusters also reset, meaning for the first few minutes into a trip, it can seem like you have much more range, or more economy, because that average has been discarded.
I have been slowly digging into the manufacturer programming modes for other things, and there is a lot of copy-paste stuff going around where different car manufacturers use the same brand ECU's etc, And some much more scary things that could be used to make things seem more economic, e.g. most european instrument clusters, even if they use an analog fuel signal, has a test mode where you can set it to read a certain value via can. same for the range and economy displays, without the vehicle getting upset you can lie to the driver, You can alter the scaling of the RPM gauge, to make it seem like your running at lower RPM, and stuff I really would never want to see one of these device touch, shift the gearbox switching points dynamically.
As for security, some have recently started using magic tokens to enter these programming modes, this to me seemed to be spurred by the Airbag Controllers before anything else, but I feel that was in response to people resetting them after an accident to sell write off cars. however per year model these tokens seem fixed, Its only really the indentured repair manufactures, e.g. certain tractor companies that are doing things like signing firmware updates and making it a right pain to enter the programming modes, (VS scattered CRC for blocks in most vehicles)