|Mathwei ap Niall|
Not sure where you're getting that their Int would drop to - as that is not a swarm trait (though it's often a trait of the creatures that make up a swarm).
Their immunities even call out that they are vulnerable to mind-affecting effects if they have intelligence / hivemind...
** spoiler omitted **...
It's not the swarm type that removes their Int, it's the vermin type that does it.
Traits: Vermin possess the following traits (unless otherwise noted in a creature's entry).
Mindless: No Intelligence score, and immunity to all mind-affecting effects (charms, compulsions, morale effects, patterns, and phantasms). Mindless creatures have no feats or skills. A vermin-like creature with an Intelligence score is usually either an animal or a magical beast, depending on its other abilities.
Darkvision 60 feet.
Proficient with its natural weapons only.
Proficient with no armor.
Vermin breathe, eat, and sleep.
As a normal Worm that walks it has a defensive ability referred to as it's hive mind which lets it think and continue to function as an individual. When it discorporates it loses all of it's defensive abilities which includes it's hive mind ability so it goes back to the default for it's type which is Vermin (it specifically doesn't get the swarm type, it stays vermin) and all vermin have the mindless trait.
A WTW who discorporates loses it's Int score and personality and becomes a non-threat.