So yeah, first, I'll argue why the Original One (True Form Arceus) should keep all abilities and moves. First, let's take a look at "the Original Story".
- "In the beginning, there was only a churning turmoil of chaos. At the heart of chaos, where all things became one, appeared an Egg. Having tumbled from the vortex, the egg gave rise to the Original One. From itself, two beings the Original One did make. Time started to spin. Space began to expand. From itself again, three living things the Original One did make. The two beings wished, and from them, matter came to be. The three living things wished, and form them, spirit came to be. The world created, the Original One took to unyielding sleep."
⠀⠀—
The Original Story
This already confirms that Palkia, Dialga and the Lake Trio are nothing but smaller parts of Arceus itself. Thus, Arceus is actually the entire Universe, reality, and all Pokemon. They are all smaller parts of the Original One, thus, this already gives us a reason to make True Arceus scale to them, since they are it. There isn't the need for the lore to go out of its way to showcase Arceus doing literally everything to satisfy the burden of proof. There is no anti-feat.
'Well, Arceus can't learn the TM Focus Punch'.
Let's look at this dialogue in Pokemon Platinum.
- "A Pokémon is said to have shaped this world. Could that Pokémon be the physical form of the original spirit?"
⠀⠀—
Hiker
Implying that the Arceus we see in game is just a physical manifestation of the Original One, an Avatar. So Arceus not learning a TM isn't an Anti Feat for the Original One, but only it's Avatar.
This should already give us a decent basis to scale True Arceus to all Pokemon. But -
Diving into the plates
So yeah, Staff seems to believe that the Plates are a weak argument. But that's only because people did a really, really poor job at explaining them. Let's first state what the plates even are, still looking at dialogue from Platinum.
- "The Original One breathed alone before the universe came. When the universe was created, its shards became this Plate. The power of defeated giants infuses this Plate. Two beings of time and space set free from the Original One. Three beings were born to bind time and space. Two make matter, and three make spirit, shaping the world."
⠀⠀—
Hiker's Book
So, yeah, Arceus' shards became the plates.
They are stated to be part of Arceus' Body.
- "生まれてくる ポケモン プレートの力 わけあたえられるPokémon born with the power of the plate."
⠀⠀—
Hiker's Book (Japanese Version)
Pokemon are born with the power of the plates, which are just Arceus itself.
Thus, saying "Well, The Creation Trio and The Lake Trio had types before the plates", I mean, yes, obviously. They are literally just a smaller piece of the creator of all types, Arceus, who became the plates. It's not a contradiction.
True Form Arceus should
not lose its ability to use all moves and abilities, since all Pokemon are just parts of him. Furthermore, the plates themselves should also not lose said capability, because they are just Arceus.