Indeed, swiveling one's head in real life does induce some lateral swing of the eyes. Simply because our head is not built as though an eyeball sits atop a stick, with just the eyeball swiveling in place.
I've never tried the game without head tracking gear since 4.08. But now that you mention it, in 4.12 I've noticed that when going to the gun sight view (SHF F1), when swiveling my head I now see this parallax effect due to modeled eye swinging that was not present pre-4.10.
That notwithstanding, no matter how the pilot"s eye might move, a collimated sight, by the very principle of optical collimation, necessarily eliminates parallax at optical infinity--and essentially so at distances larger than about 100X the collimating lens diameter. Recall that for, say, a 5cm diameter collimator, the eye point lateral shift of 5cm across its width results in a parallax of 1 mil at 100 * 5cm = 500cm = 5m. The game's use of a typical ~10m for the distance of the reticle would then exhibit a 0.5 mil parallax for an eye point lateral shift of 5cm.