This will be the first time the Black Manta is going to the Spratlys so there is not much information I know about the diving, beyond Layang Layang. The other places it is planning to visit are not that far away from LL so I believe the diving is similar but without the crowds. According to
Welcome to Whitemanta.com,
7 Days / 6 Nights Layang Layang and The Borneo Banks Exploration (Sprartlys)
Borneo Banks are beyond Layang Layang and consists of the Ardarsier shoal, Mariveles reef, Investigator Shoal, Erica Reef, Dallas Reef. Sailing time is about 17 hours from Kota Kinabalu.
Price SGD$2500 (~US$1,800) per diver in twin or double ensuite cabin
Price Includes
Airport transfer from KK airport to Boat and to airport
6 nights onboard Black Manta (5 full days of diving)
All meals, drinking water and hot berverge, snacks and fruits
Towel , daily cabin services,
Tanks, weight belt, dive guide
Port fee
I don't know about the boat you found but that looks like a decent savings if it is anywhere close to the Black Manta standards of service, comfort and safety.
In the interest of full disclosure, I sometimes work on the Black Manta as an instructor.