There's a plethora of people who convert to religion at an older age, and that seems far more far fetched than Santa.
Being in a religion doesn’t imply belief in deities; it only implies people want social connection. This is clearly visible in global religion statistics; there are countries where the majority of people identify as belonging to a religion, and at the same time only a small minority state they believe in a “God”. Norway is a decent example that I bumped into just yesterday. https://en.wikipedia.org/wiki/Religion_in_Norway
But I bet you'd have a significantly easier time converting a child rather than a 30/40/50-yr old to a religion.
My point is that LLMs are suggestible, perhaps more so than the average adult, but less so than I child I suspect. I don't think suggestibility really solves the problem of whether something has AGI or not. To me, on the contrary, it seems like to be intelligent and adaptable you need to be able to modify your world model. How easily you are fooled is a function of how mature / data-rich your existing world model is.