Data Sphere

From the pdf:

The trooper is vulnerable to a type of Attack or Special Ammunition
listed between brackets in its Troop Profile. Owners possessing any
kind of Immunity Special Skill cannot apply that Special Skill when
facing such an Attack or Special Skill.

This seems to be a benefit from a model that imposes a disadvantage on another model, contrary to what one would normally expect.

e.g. a model with "Vulnerability: Armor-Piercing" would impose this on a target and negate that models "Total Immunity".
Am I reading it correctly?
I would initially suspect it to cancel a benefit on the model it comes with, basically if it has "Total Immunity" and "Vulnerability: Armor-Piercing", it would still suffer halved ARM vs AP ammo, but still retain the "Total Immunity" vs everything else.

Can someone enlighten me?

It is simply a way of making the rules simple.

Before, the Total Immunity had gaps in it, like not being applicable to Viral Ammo. But you had to read into the skill's description and remeber this nitpick.
Now, Total Immunity is Total on its own. Any openings in it are noted separately, as Vulnerabilities.