They didn't mean it as a pun, but understanding it as a pun helps understand the situation.

In religions, missionaries are those people who spread the word of god (gospel) as their mission in life for a reward in the afterlife. Obviously, mercenaries are paid armies who are in it for the money and any other spoils of war (sex, goods, landholdings, etc.)

So I guess he's trying to frame it as them being missionaries for an Open and accepting and free Artificial Intelligence and framing Meta as the guys who are only in it for the money and other less savory reasons. Obviously, only true disciples would believe such framing.