It means that in handheld game already struggles with 60 FPS in battles and by using docked mode you are making it only worse and you won't be able to get something stable at all.
How "not recommended" can mean "can't use it"? If it would not work, I would write "it doesn't work in docked mode".
Once again I fell down the martial arts rabbit hole and am plumbing the murky depths of the internet's various martial arts and fighting crafts echo chambers.
If you're not actively involved and only get up to speed every few years or so, you can see the tide of public opinion flapping worse than my dong while I'm skipping naked down a hallway.