I wasn’t in front of my computer at the time.
It should be
from pyrevit import HOST_APP, UI
uiapp = HOST_APP.uiapp
command_id = UI.RevitCommandId.LookupCommandId("CustomCtrl_%CustomCtrl_%CustomCtrl_%Add-Ins%SPICETOOLS%RUTILITIES%Librarian:Librarian.ExternalCommands.CommandToggleDockVisibility")
uiapp.PostCommand(command_id.Id)