The 40-second play clock will apply to everyone, visible play clock or not. Colorado, Michigan, Tennessee, Indiana, and anyone else who experimented with a 40-second play clock, did not universally have visible play clocks.
What was intended to be said in the document was that the play clock operator would react to the specified signals (dead-ball signal, stop-the-clock signal, or incomplete pass signal) as his cue to start the 40-second play clock, or to the pumping of one hand to reset the play clock to 25 seconds (or of 2 hands, to reset to 40 seconds).
In the absence of a visible play clock, the officials themselves would start a 40-second play clock following the appropriate signals in the appropriate situations (dead-ball signal for a running play that ends inbounds without the game clock stopping, stop-clock signal for a running play that ends inbounds with a game-clock stoppage (first down gained), or out of bounds, or incomplete pass signal for an incomplete pass). The back judge (or other official responsible for the play clock) would also act on the one hand pumping and two-hand pumping signals from the Referee (the 25 and 40 second rests, respectively).
Because the people who write the rules for NFHS apparently do not have a proper command of English grammar and/or style, the words on the paper get butchered to mean that the 40-second play clock does not apply if there is no visible play clock available, which is now contrary to the 2019 NFHS football rules. The officials are, and remain, responsible for the play clock when there is no visible play clock, whether the play clock will be reset to 25 seconds, as it always had been before, or to 40 seconds in some situations and 25 in others, as it has been in the NFL and NCAA for the past 10+ years, and it will be in NFHS starting with the upcoming 2019 season.