This is a tough one. I've said before that classification can't usually be determined from the NEC itself - except for installations subject to Articles 511 to 516. Where they apply, those Articles basically preempt other classification documents .
In this case, Section 511.3(E)(1) would be relevant but it doesn't answer your question directly unless you use ventilation or positive pressure. Unfortunately, a "... self closing door with a tight seal ..." still isn't a wall or partition.
Even more general classification standards, such as NFPA 497 or API RP 500, wouldn't recognize a "... self closing door with a tight seal ..." either; however, they would typically create the envelope based on the source of release. The problem with Article 511 is the "source" is the "floor area" described in 511.3(C)(1) or 511(D)(1) and the office would simply be treated as a extension.
That said, I personally would go for ventilation. It isn't that onerous.