The 3.7 patch notes indicated that this has been fixed - “The special event rules in Across Ixalan and Battle of Four Tribes no longer count as Spell cards, and do not trigger multiple effects anymore.”
This has not been the case so far. It’s popped Insidious Wills and triggered Mizzium Tank. In the battle log, it’s still being displayed as a spell that’s being cast and triggering effects that care about spell casts.
//Edited Title - Tombstone