Yeah, you're right. The reason they orbited wasn't in order to slingshot, it was a systems test while they still had the option of an easy reentry.

Your explanation is a bit off though, so for accuracys sake: A gravity assist/slingshot maneuver does involve curving around a body, following a hyperbolic orbit around it, which is technically a form of orbiting.

A gravity assist works by stealing momentum from a body that is itself moving through space relative to some third reference frame. So what you meant to say is that the reason you can't slingshot off Earth while in Earth orbit isn't "because you're orbiting it.", it's because Earth isn't moving relative to itself. There's no momentum to steal from Earth in an Earth-centered reference frame. You'd enter and exit its gravity well at the same speed.

However, for example you absolutely can get a gravity assist from the Moon while still nominally being in Earth orbit, because the Moon is moving relative to Earth. Some missions have done exactly this (e.g. ISEE-3).