I have not seen many scripts use this kind of thing:

set QuickCellRef to player.GetParentCell
if ( player.isIninterior == 0 || GetCellBehavesAsixterior QuickCellRef == 1 )
message"this place seems to be outdoors."
endif

But it would seem that only using player.isIninterior == 0 is entirely not good enough.

But that is all the original developers had when they made scripts.

How was it that their script worked with out the OBSE GetCellBehavesAsixterior command?

My first thought would be that there are only a few Cells which use the Behave as Exterior value (if memory serves-- I can only think of the Arena, Oblivion Sigil Towers, and possibly Camoran's Throneroom in Paradise, but there may be more), so they just relied on Hardcoded effects when dealing with these: i.e., rely on the Hardcoded Sun Damage effect so Vampires burn in the Arena, but not in the Sigil Tower.

Although, I have not really looked at Vanilla Scripts for a lot of things, so if you had something specific in mind, that might be totally off the mark

Is there a vanilla script used to detect exteriors ? Had the devs felt a need for such functionality, they would have added a script command that would've provided it. The isInInterior command simply checks for the Interior flag on the the calling ref's parent cell, which should be set when it doesn't have a parent worldspace. So it's reliable, as long as exterior cells need a parent worldspace (don't they?)