I can totally understand not really wanting to engage with these stupid people but to suggest that it doesn’t matter that he believes that is disingenuous.
If he’s stupid enough to think the Earth is flat then he is stupid enough to do other things in his actual job wrong or in a dangerous manner.
But isn’t the argument about critical thinking skills? I’m sure it’s nice to believe in Gaia but there is demonstratively no evidence for it.
The question of harm done is independent to that of gullibility.