There is definitely a worm gear and a clutch inside, as reported by the (now defunct, but luckily archived*) TechnicBricks blog:
The orange part is the input and engages with its serrated surface the pins of the white cylinder. The flexibility of these pins allows the rotation of the input axle while the actuator shaft is blocked - providing the clutch functionality.
The inside of the white cylinder has grooves in it which then engage and drive the worm gear structure of the actuator shaft - provided the end of the shaft is prevented from rotating. Otherwise the cylinder and shaft just rotate together.
The Eurobricks forum has a thread detailing the differences between the various iterations of the linear actuator, with the following images of the internal components as they got updated based on customer feedback:
* Have you considered donating to the Internet Archive?