There are indeed lots of minis carrying thunder hammers or leaning on them with one hand. That doesn't make it a one-handed weapon.
I can carry a chainsaw in one hand myself, and would do so. but I need two hands to use it. Look at the size of the thing. It just looks silly to use it in one hand to me. And I'm from the era where these are strictly terminator weapons. I can live with them being used by 'mere' power armour in two hands, but not one.
As regards players pulling stuff like 'that', it's why I mentioned the 'and isn't some unique relic or exception' caveat. Those are being worn by a Chapter Master, and no other mini uses such a thing. So players showing me a picture and saying 'see, now I can have one' does more harm than good really. Because it gets the answer 'sure, when you have the renown of a chapter master and ask me to write the relic rules for such a thing!'.
40k has had about 25 years to build a universe and themes, with literally thousands of minis setting the 'tech level' of the game nicely and displaying what can and can't be done. So it's quite useful to use them as a guiding start point for vetoing munchkin or outlandish suggestions.
I can do maths: Twin shoulder mounted hand flamers would rock. And it'd look cool. But it exceeds what has been repeatedly demonstrated to be shown in the universe. Plus: If players want to hang a load of chuff off their armour, they should go and play Tau!