I don't think there's a single "comes into play" event. Mirror Entity obviously copies minions after their Battlecry has finished triggering so you can get more than 4/1 Twilight Drakes. Maybe it straight up triggers off the "battlecry finishes" trigger so that can happen. There's all kinds of states of being in and out of play.
I think he's saying there
should be a consistent one.
To rephrase it (mostly for myself): A scientist killed by a battle cry ability can grab a mirror entity and have that mirror entity activate
for the minion that killed him, meaning his deathrattle effect is going off specifically
before the minion comes out, otherwise entity wouldn't copy it.
When Sylvanus is killed by a battle cry, her deathrattle seems to wait for that battle cry minion to come into play, then her deathrattle activates and steals it, contrary to how Scientist works (where the deathrattle happened first, then the minion came into play).
It's less about the battle cry and more about the deathrattles in this case. The Mad Scientist and Sylvanus interactions at at odds with each other, even though they both trigger on deathrattle.
I agree that generally speaking, any time something "coming into play" is important it's usually referred to as a "battlecry", but with this example we're seeing a rules inconsistency that should be tightened up one way or another.
FAKE EDIT: Actually I think your example brings up something else in regard to battlecry effects. I guess they always happen before the minion hits play, even if it's buffing itself? That seems weird to me. They would have to, for entity to work, unless it's a special case.