Arena's rule engine was explicitly not based on Duels; this was a big selling point when it first launched to replace Duels, and they've gone into some detail s

Arena's rule engine was explicitly not based on Duels; this was a big selling point when it first launched to replace Duels, and they've gone into some detail since then describing how it works. See https://magic.wizards.com/en/news/mtg-arena/on-whiteboards-naps-and-living-breakthrough https://magic.wizards.com/en/news/mtg-arena/on-whiteboards-n... for instance.