Heretic of The Seventh Circle
I’d say that DM is within the rules, but only because summoning a thing out of thin air is clearly magical.I don't argue that the Sage Advice version of it isn't a decent take. It is fine.
But, I also think a GM with the PHB and DMG in hands would be entirely okay to say, "No, you can't summon that thing in an anti-magic field, because it is magic." And not be outside the rules in so doing.
It is a "rulings, not rules" thing.
They’d be outside the rules if they said it disappears when you enter an AMF, though. I’d have to reread the ability more closely to say if it’s properties are dampened like a normal magic item, or if they seem to be more like a dragon’s breath or flight.