It's there, but I don't think you'll find a statement that says, "if it's not a voice remote it won't work with sticks". For the "simple" remote it states:
"Compatible with
Most Roku players* and Roku audio devices.
Not compatible with Roku TV models.
*Except Roku® Streaming Stick® models: 3821, 3820, 3811, 3810, 3800, 3600.
To check if your device is compatible, find your device model number."
If you compare remotes, there will also be a "compatibility" category with the same information. There is a caveat: there are very old remotes that are not voice remotes that will work with sticks. However, for current remotes, it must be one of the voice remotes to work with a stick.