If they were doing their AOW, then they would be bringing an instructor with them. For guides, perhaps they have their own that they're comfortable with (language, as posted)? Sometimes, dive ops start consolidating trips to dive sites during low season. One boat will host groups from multiple dive ops.
I haven't done diving from the Phuket area, but twice from Koh Lanta. The dive sites, Koh Ha and Bidas, are over an hour away by boat, which means that the gear set up is really relaxed. (You have at least 45 minutes after breakfast!) The dive ops I've been with Andaman Dive Adventure, Lanta Divers and Koh Lanta Diving Center, are full service, in that they'll set up the gear for you; all you have to do is dive. Being a new diver, I would suggest that you ask to set up your own gear with supervision, just to better your set up skills, so it'll be second nature. Andaman Dive Adventure is the smaller of the three outfits, so the boat isn't packed.
Here's a typical day out:
~7-7:30 AM free pickup from accommodations
~8 AM board boat and depart
boat briefing
breakfast
relax, chat with other people, set up gear
~9:15 AM dive briefing
~9:30 AM 1st dive
~10:30 AM surface interval, fresh fruit, cookies
~11:30 AM 2nd dive
~12:30 PM start heading back, lunch is served
~2-3 PM dive logs and payment at dive shop
drop off to accommodations
There are dives out to Hin Daeng and Hin Muang, which will add an extra hour earlier to get there, i.e. 6-6:30 AM start, and an hour later, i.e. 3-4 PM finish. Hin Daeng and Hin Muang are deeper sites, prone to currents, so either have AOW or dive a few days first, to let the dive op check you out, to make sure you're good with buoyancy and air, before attempting on OW certification. The appeal of Hin Daeng and Hin Muang are mantas and whale sharks and now is indeed the season for them. There's an extra fuel surcharge to these sites due to the extra distance, in case you're on a budget.
From Koh Lanta, there are ferries that will take you to Phuket, though I haven't researched. Flying would require backtracking to Krabi, then flying to Phuket.
From Bangkok, you can get to Koh Lanta via a few airlines. Back in April, I flew NokAir to Krabi from DMK, then took a shared minivan to Koh Lanta. The minivan was 600 THB, direct from the airport to your accommodations. I didn't need to book ahead, just arrived, picked up my bags, and there are quite a few travel agencies right there. The first minivan will be pretty nice, then you will be taken to a depot where you'll be switched to another minivan that's not so nice. The ride can be fine or really cramped, depending on how many people go at that moment. It's about 1.5-3 hrs, depending on traffic.
To get to DMK, you can take a taxi but an airport link might be operational now. It was under construction back in April.