Invulnerable saves have the same issue as cover saves. The only time they describe a vehicle making a save of any kind, it's in response to a glancing or penetrating hit. This is the first time they've tossed in an attack that doesn't produce glancing or penetrating hits, and the rules twist oddly as a result.
Well, it's not really actually. And there's even one "attack" objuration mechanicum, that isn't an attack proper, it's a malediction, and it causes glances and pens, yet you don't get cover saves against.
I kinda wish you'd address the logic of an effect occurring at the "hit" stage, such as entropic strike, mindstrike, marker lights, grounding tests, versus things that occur at the "damage" stage, such as haywire, concussive, Gauss weaponry, and I think, this.
There's a fairly clear precedent that things that occur on hit, such rad grenades, or even JOTWW you don't get saves on, very things that occur on dmg, like psychic shriek, you do (even if it ignores cover, you get any invo saves)
Moreover can I point out that Grav weapons certainly are causing damage, in the traditional way, even though they're not specifically causing pens and glances, while entropic strike actually does not? Basically I think you're looking at this from the wrong end and thus using the wrong set of precedents.
If you need even more backup, may I point out that GW uses several different phrases to refer to "taking a wound" as well as several for "removed as a casualty". They just don't use keywords heavy enough to really on "glances or penetration hits" language.
