Even if the grammar doesn't convince you, the rules work a lot better and you avoid opening a big can of worms if you play that it's only a psyker for the specified purposes
Lets be honest here, there is zero Worms, and if there is a Can it has a resealable plastic lid--its not even child locked.
If my vehicle is a psyker, what issues would we possibly have? Mind strike missiles cause glances, big deal--we already know how a glance interacts with a vehicle. The crucible of malediction may kill a tank or two, maybe as much as 9 if you really go out of your way to make a circle around the CoM and fail a bunch of
LD checks. As we all know, removing a vehicle from the table is not such a hard thing to do, we do it every time we explode one. Draigo would be s10--ask possessed vehicles if this causes an unresolvable situation that breaks the game.
As for the Culexus assassian, if you really want to get a ton of shots you already can. I have played versus grey knights quite a bit, and the Culexus with 20 shots if and only if you center your entire army around it is a laughable novelty. Strike squads get 20 s5 shots already, for less than the cost of using your entire armies transport capacity in one place, and you dont even see strikes in that config often despite the total entry cost being 220 as opposed to 500 the proposed culexus with rhino spam would be.
Plus, if you follow the logic that vehicles can not be psykers because a single piece of anti psyker wargear would then be useful--the crucible, that would mean that warp quake is clearly being played wrong because it can table a daemon army without even trying. Face it, some things in the game are unbelievably good at killing niche unit types like daemons, deep strikers, or psykers.
So yeah, suspected balance issues with a vehicle that is a psyker is not a valid defense of this rule.
Really the grammar is what matters. The proposed reading of the sentence "A vehicle with this special rule is treated as being a psyker the purposes of Psychic tests and psychic hoods" makes no sense. Counting as a psyker does nothing for psychic hoods, psychic hoods work on models (as opposed to psykers) that use a power.
In addition, as pointed out, this reading of the rule does not make the vehicle a psyker for the purpose of psychic powers, only psychic tests. "A psyker can use one psychic power per turn." According to the reading proposed above, the vehicle would only be defined as a psyker for psychic tests--so you cant use your powers.
Thus, if you read it such that the vehicle is a psyker, then it is a psyker all the time and its powers work.
If you try and use the grammar to claim that it is NOT a psyker all the time, then the same grammar structure optionally chosen to be used also prevents the vehicle from using a psychic power.
Obviously, we must use the grammar that allows the vehicle to use its power right? We are not advocating that Fortitude can not and should not ever be allowed to be cast due to differing grammatical readings of the rules sentence are we?