if you have a direct confirmation from TG you should be on the safe side
sometimes OLCI does not work if they are not clear about your nationality and visa status (resp. if you qualify for a visa exemption entry)
even there was a hiccup and one of your two flight is full (unlikely as it is still low season and tourism figures to Thailand are down), there are several flights per day of TG for HKG BKK and BKK HKT
let us know what happened!