Dude the effect doesnt happen on the summon, it applies after the summon. Its not a continuous effect which applies when it hits the field.
The effect only activates once it has been successfully synchro summoned.
Taken from Wikia:
"One of Arcanite Magician's other useful features is that he only has 400 ATK as soon as he is Synchro Summoned to the Field. Because his ATK boost is due to his effect, and happens after he is Synchro Summoned, he is immune to Bottomless Trap Hole."
This is all about basic game mechanics, that an ignition effect occurs after the summon.
Take this example. You tribute summon Raiza the Storm Monarch and target their facedown monster. Your opponent responds with Bottomless Trap Hole.
Look at the example and you should see that it is obvious that Bottomless Trap Hole resolves before Raiza's effect resolves.
Taking this into consideration about Arcanite Magician, Its effect wont resolve until after the summon. Not during it, so its attack will be 400 and cannot be bottomlessed