Booking ETS Platinum class trains more than 24 hours in advance

I'm looking to book tickets on an ETS Platinum train service from Kuala Lumpur (KL Sentral) to Ipoh. The official timetable shows a number of Platinum services (EPxxxx) running daiy; however, while other ETS services show up on booking portals, none of the Platinum trains do, even on the official site.

However, this random site says that Platinum bookings for the KL-Ipoh sector open only 24 hours before departure. I can't find confirmation of this in any even semi-official source. Is it really impossible to book Platinum tickets more than a day in advance? (And if so, why!?)