score:6
CTM's website, as you link above, is happy to show me schedules, prices, and a booking form for trips from Essaouria to Sidi Ifni (with a change in Agadir) for next week. They may simply not have loaded, or even decided, the schedules for the dates of your planned trip yet, if it is several months in the future.
According to the PDF schedules on the ONCF website, Supratours Agadir/Laayoune service does not visit Sidi Ifni; instead, it goes via an inland route, passing through Bouizakarne and Guelmim. You could consider getting off the bus at Tiznit and taking a grand taxi there, but given the existence of the direct CTM service, this doesn't sound like the best way to go.
All of this is, of course, subject to change at short notice, and is best reconfirmed upon your arrival in Morocco if possible.