[Antonello]

I was initially looking at New American, but the other two -- the two most widely accepted versions -- clearly say he'll die that very day.

It looks like from Theo's original quote that he uses the English Standard Version. Here's Gen 2:17 from that one:

"but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat[a] of it you shall surely die."

Be aware that the [a] footnote in this version offers up 'when you eat' in place of 'in the day that you eat', apparently because someone took it upon themselves to edit God's word to remove the problem of a literal day reference.

[Theo]

You're saying the translators of the ESV are intentionally mistranslating here? They're "editing God's word"?