I'm probably just overthinking this, but...
Steven's Aggron has a very odd moveset in Ruby, Sapphire, and Emerald. It has no Steel- or Rock-type attacks, instead running the moves Earthquake, Thunder, Solarbeam, and Dragon Claw. On one hand, I understand why it doesn't have STAB moves; its only Rock attack in Ruby and Sapphire was the weak Rock Tomb, and its only good option for a Steel move would be Iron Tail. What I don't understand entirely is why Game Freak decided to almost entirely eschew physical moves for it, instead giving it three Special attacks to run off of its extremely mediocre base 60 Special Attack. Its Physical movepool in RSE was a bit limited compared to its Special options, but a set of Double-Edge (with Rock Head), Earthquake, Iron Tail, and mayyybe Rock Tomb seems like it would be serviceable enough.
While thinking about this the other day, it occurred to me that Thunder and Solarbeam are moves bolstered by weather, the very weather conditions that Kyogre and Groudon are known to summon. Conveniently, they are also weak to those attacks. Looking at Dragon Claw, Aggron is one of a relatively small number of non-Dragon Pokémon to learn the move, and as a Dragon move, it's super effective against Rayquaza, the third weather-manipulating legendary in Hoenn (Aggron also gets Ice Beam/Blizzard which would be better against Rayquaza but I'm ignoring that for the sake of this argument). If it was actually the intention for Aggron to know these moves to counter the titans, it ignores the unfortunate fact that Aggron is weak to Water and Ground, and is slower than the legendaries, meaning it's (probably) not going to get a chance to attack against them before dying.
Of course, it's also just possible that it knows Thunder and Solarbeam to help cover it's general weaknesses to Water and Ground, but I found the fact that Game Freak chose those weather attacks interesting.