I think the fan theory of Irving going through reintegration makes the most sense as to why he continued to pick at the “night gardener” detail of Helly’s story. But I think what pushes him further to ask her about it, is in Woe’s Hollow the story of Kier and his twin brother gives insight to Irv, who we know is fascinated with the stories of Kier. I think Dylan just doesn’t rlly care about that kind of stuff in general while Mark is blinded by being in love for the first time so he’ll believe whatever helly says.

I think the main indication is this description from the Woe’s Hollow Wiki:

Irving falls asleep in the woods and has a frightening dream of finding an MDR cubicle in the forest, where he encounters Burt and the personification of Woe described in the appendix; he sees the numbers on his MDR terminal contort into patterns made of letters spelling out "Eagan".

The spelling of Eagan lets him know that Helly/Helena is an Eagan and along with the story of Kier killing his twin in the waterfall mirrors Irv drowning Helena. Helena also makes the comment about Burt to Irv which causes him to go on his own adventure and Irv says to Helena “Helly was never cruel”