Does a "may digivolve this card as if the Tamer is a Digimon" effect also treat the Tamer that will become a digivolution card as a Digimon when it digivolves?
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Yes, it also treats the Tamer as a Digimon when it digivolves. It treats it as a Digimon that digivolves, therefore "when a Digimon would digivolve" effects and "when a Digimon digivolves" effects will trigger. In addition, if a "Digimon can't digivolve" effect activates, digivolution won't be possible using this effect that also treats the Tamer as a Digimon.
Source: BT7-071 Loweemon