Lets's use an iphone as an example. In the past if it was turned off there was no access to the data. Now that data is synced to the cloud. My linux computer syncs to the cloud. Now I have devices that were formally secure if off that can be compromised now because the data has been uploaded to a server on the internet.
I know what cloud means. Was that example simple enough to understand?
ETA: Technically you're correct as the device can not be compromised if there is no power but we're really talking about data and you're splitting hairs.
