I'm fond of gaseous, but I could go either way as long as we're consistent about it. Just because the original text had problems making sense doesn't mean we can't make sense!
I prefer incorporeal for them, with them manifesting in a gaseous form to make an attack.
Here's another idea, perhaps the "insubstantial state" is effectively a Hazard, which transforms into a creature with some kind of template if it successfully attacks. In mist form it has very few vulnerabilities or attacks, so can easily be written up as something like:
Transient Golem (CR X)
A transient golem is a strange entity that appears as a whisp of mist, a DC20 Spot check will notice it is actually a being with vaguely humanoid features rather than a natural feature. When in fog or mists these entities are effectively
invisible. Transient golems fly at 10 feet with a perfect maneuverability and can sense any living creature within 60 feet (as per the
detect animals or plants spell, except it detects any living creature).
Transient golems are naturally
incorporeal/gaseous (see gaseous form} [to be decided] and are immune to all forms of damage and magic apart from spells with the Abjuration descriptor which can block or banish the golem. A transient golem's life-force is contained in a crystal phylactery (see Manifest Transient Golem Template, below), any effect that would destroy, imprison or banish the golem instead sends it back to its phylactery. Abjuration spells always have their full effect a transient golem regardless of their descriptors, e.g. a
dispel evil spell will banish the golem back to its phylactery, even though it is not Evil, and a golem can not touch a creature under a
protection from good spell, even though it is neither Good nor summoned.
A transient golem resists Abjuration effects as if it has 13 Hit Dice, 16 in all its attributes and +7 Fort, Ref and Will save.
Transient Golems attack living creatures by manifesting part of their body as a tendril of mist and trying to insert it into an orifice of their intended target. [we can copy most of the previous Infusion attack for this.] If the attack is successful, the golem flees and, sometime within the next hour, it can solidify its body into a copy of the creature it attacked (see Manifest Transient Golem Template, below).