As @holbrook says, it isn’t stated, that was just a list to show possible ways to do it if you wanted some kind of field control. Since you never switch between modes (disable/enable, driver/autonomous) in Skills, there doesn’t seem to be anything to say that you couldn’t just start a driver program, and wait for whoever is doing the timer to give you the go ahead before you drive for example.
The manual says
Please note that all rules from “The Game” section of the manual apply to the Robot Skills Challenge, unless otherwise specified.
And G8 says
<G8> Controllers must stay connected to the field towers. Prior to the beginning of each Match.....
But a match is specifically defined as
A Match consists of an Autonomous Period followed by a Driver Controlled Period for a total time of two minutes (2:00) so therefore not a skills match.
And that seems to be the only time that Field Control must be used.