Well the pharmaceutical company being involved is a bad sign that this is some kind of fucked up experiment...
It could be.
Also, the appearance of Seong Su's son and the man he's with seem to reinforce the Trinity symbolism. There's the father/God (that CEO/leader), the son (Seong-Kyeong), and the holy spirit (Ryeong, with the dove). That leaves Seong Su with the implication of being Mary. This all feels too obvious to be the truth...
I'm still inclined to think of it as an extra sinister human trafficking ploy. Backed by a pharma for reproductive and hallucinatory drugs and tech, administered to the chosen followers to keep up the "miracle" narrative that draws in more members. But who knows, maybe the author decided to go the science fiction route.
I still don't believe this is mpreg, especially not in an abo/natural reproductive phenomenon framework. There would be physical signs that Seong Su has given birth either naturally or by C-section, both of which would result in varying degrees of scarring. Or even his anatomy being at all different (self lubrication, different reproductive sects that could be identified by anal fingering) would be included by the author who takes a more realistic approach in these details.
Unless it's some medical technological advancement that allows men to get pregnant and he gets superb reconstructive surgery afterwards, I fully doubt this is the case.
All the "signs" of mpreg read to me as red herrings, as it's perhaps part of the manufactured divine miracle that enables the cult leaders to amass influence and followers. Which then raises so many questions, I'm sure it will be a complex web of machinations. I wonder to what extent Seong Su has been influenced by them, does he believe in any of it or is he fully aware of the manipulations and lies and is coping because he wants to save his kids?