It seems that “epistemic humility” is rather the antithesis of “I decided against belief in the supernatural."
In context with the above, it also appears that “I think the only safe way to handle God conceptually is as a mystery” still precludes whatever “God” is from being supernatural, so the antithesis with epistemic humility remains.
“I recommend asking the mystery what it thinks” is also antithetical to it because it is stated explicitly that the supernatural God has already revealed is excluded, since “Everything some dumb kid dreams up or which is written in a holy book or explicated by a scholar is necessarily speculation.” Where is this hypothetical humility please, with “some dumb kid” on a par with Bonhoeffer’s hermeneutic?
(I took the liberty of italicizing your I, @Terry_Sampson, because your “I could be sure” was not indicating a possibility but a certainty, since you followed it with the declaration, “I’d stake my life on it.”)