Hmm. Interesting. Dakkadakka has a big thread on this, here is what was posted. Seems like it might work
Yeah, I saw that thread and that interpretation around Reddit too. I still feel like its questionable though - FAQ worthy for sure.
I think this stems from the fact that all rules are written in non-matched play format. Specifically regarding Cult Ambush - setting up underground is effectively tactical reserves - thus turn 2 deployment in matched play.
Kind of feels like this was just intended to slightly break the 50% deployment rule - not provide turn 1 charge opportunities.
I would love to be wrong - if it is allowed that is a really good strategem.