Displays in a helmet aren't "display glasses" because they're not glasses.

Display glasses are a subset of HMD's, a more specific term. The "display" part of it implies that they're monitor replacements -- both in their interface (DisplayPort alt-mode) and an FOV suitable for monitor replacement.

Finally, being a monitor replacement implies a mostly-2D view, in contrast to XR/VR/AR which implies 3D.