I think the core question here is a little more complicated that y'all are making it out to be.
As to whether a weapon which is also an implement can benefit from Weapon Fcous, that seems clear.
But with weapons like the Holy Avenger, that is not what is happening. The Holy Avenger (and other weapons of its class) says that it "can be used as a holy symbol." You could read that to mean that, when using it as a holy symbol, it is no longer a heavy blade or whatever, because a holy symbol isn't even a weapon.
I'm not sure I agree with this, frankly. I think it unduly punishes split weapon/implement classes. But it is (as far as I can tell) a valid interpretation of the rules.
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gnfnrf
Well that depends on what 'Weapon Focus' represents.
See, the thing about 4th edition, the funny thing about it, the thing that makes it 4th edition, is that powers, even -weapon- powers, are not always strictly martial displays of strict weapon techniques. And Weapon Focus doesn't care.
So, you can say 'Weapon Focus is only strict use of the weapon as a weapon and nothing more' but then you forget that 75% of the classes in the game
do not actually work that way.
Take the Paladin as an example. He's not a strict 'I use a sword and it is how I do things' character, he actually infuses his attacks with an aspect of divine power in greater or lesser degrees. It's hard to justify that a character who can mete down the judgement of the gods to enforce one-on-one combat, can infuse his attacks with holy power that literally channel light itself into a deadly blow, and can combine such into a blow that makes an enemy less able to strike true with his weaponry, does so sheerly through the training inherent in weapon training. (Divine Challenge, Holy Strike, Enfeebling Strike)
In such a case, Weapon Focus would represent -for that paladin- the ability to use a sword to more effectively channel divine wrath. Does some physical weapon play involve itself in that? Yes. But I would hesitate to say or even infer that -any- feat that boosts a paladin's powers would be ONLY strict martial weapon play.
So, in the hands of a paladin, yes, the weapon focus feat then represents using swords to channel divine wrath because that's how he does stuff with his sword. It is then no stretch to say that, if his sword is also a holy symbol, that the same techniques used to channel that holy wrath can be used when using it in a non-martial manner. He's still doing the same channeling of holy wrath, he's just not moving the blade so much.
Or, maybe he IS. Maybe his implement attack consists of him grabbing his blade, and cutting through the air, and having divine will cause that cut to go through his distant enemy.
Perhaps he is using a swordstrike so precise and accurate that he not only does so, but the ambiant light reflects off the blade into his foes eyes. Or maybe not ambiant light, but the very light of holiness that suffuses every single thing he does.
You have a game system where you have a class that uses sword-skills to teleport yards away and cut people with the power of dimensions itself; having weapon focus reflect more than pure martial prowess is hardly even a stretch.
tl;dr
Weapon Focus doesn't mean anything on its own, fluff-wise. It reflects what the -character- does with that weapon, and the ability to use it with that character's powers. A paladin's weapon focus (and all his feats, actually) are different things than a fighter's weapon focus, even if they have the same game effect.