Yugioh Pro says Slifer the Sky Dragon's effect doesn't work if Level Limit Area B is on in play. Is that right? I found zilch supporting it?

I'm guessing the part they're arguing involves Slifer's "If a monster(s) is Normal or Special Summoned to your opponent's field in Attack Position: That monster(s) loses 2000 ATK, then if its ATK has been reduced to 0 as a result, destroy it." and Level Limit Area B's "Change all face-up Level 4 or higher monsters to Defense Position."

Since a monster summoned in Attack Position would be immediately changed to Defense Position by Level Limit Area B's constant effect, the trigger from Slifer's effect shouldn't be able to activate since the target is /immediately/ changed to Defense Position and thus doesn't fulfill the requirement for Slifer's "-2000 Atk & Destroy if 0" effect because the target is not in Attack Position.

This would differ if a card were summoned in Attack Position, and /then/ a card was activated in response that changes it to Defense Position. In this case, the monster was not immediately changed to Defense Position, there was a moment where the monster met the requirement for Slifer's trigger effect's targeting.

In order to target the monster summoned, the monster must have been summoned in Attack Position. Though the summon was made to put it into Attack Position, if the monster is in Defense position before the targeting is resolved, then the targeting fails. If another card that would change its position is still resolving at the time of Slifer's targeting, the targeting succeeds and the effect will begin to resolve.

Think of it this way:

If Dark Magician were summoned and no cards were immediately acting on it, Slifer's effect would add to the effects that should affect it.

A constant card like Level Limit Area B immediately affects anything on the field.

So Dark Magician is summoned on an empty field with only Slifer present, effects acting on DM: (1): Slifer's effect.

DM is summoned on a field with Level Limit Area B & Slifer, effects acting on DM: (1): Level Limit Area B, (2): Slifer's effect attempts to target the Attack Position monster, but the monster is not in Attack Position even though DM was summoned in Attack Position initially. The targetting fails, so the effect doesn't occur.

DM is summoned on a field with Slifer and a face-down Enemy Controller (or in the hand of the turn player). In response to DM's summon, Enemy Controller is activated (not yet resolved). Effects acting on DM: (1): Enemy Controller to change to Defense Position, (2): Slifer's effect

Because Enemy Controller's effect hasn't resolved yet, DM meets the requirement for Slifer's targetting, so Slifer's effect will activate on DM regardless of whether DM is in Attack or Defense Position. It does not matter which effect resolves first.

Time to leave the basement.