Yes the front LED goes off when the screen saver is active, but the Roku is still running actively running the screen saver with video output.
A true standby mode would turn the video output off and shut down to very low power with the only activity being watching for a key press on the remote I would think.
So far I have not seen such a standby mode on the Roku Ultra 4640, only screen saver mode where the device is still active as far as I can tell.
So thats why I am using a power strip that can shut off devices completely when the TV is off if they don't have a true standby mode.
Roku Ultra 4640R, Roku Streaming Stick+ 3810R