Titans were a pretty large part of Greek mythos. They were practically equal to the Gods at one point. They were very intelligent~
The story of Marionette is fairly mythological, so it is possible. Could it be about Centaurs slaying Giants? Unlikely... but possible~
However, the "carcass of the giant," could also be some form of ecological power. Something big in the world of it's time, but not a living being. The fall of a company perhaps...
It's hard to tell; Most of the song is metaphorical. The second verse, for example, appears to be the confused hallucenations or imaginings of someone.
"Creeping on the bloodstock concubine," <-- It's hard to make sense of this as it could mean a few things. A large amount of female horses? A low-status woman who gave birth to a lot of horses? A low-status woman who is able to give birth to horses? A low-status centaur female? "Claiming you alone had caught the skyline. A chance to look, a fool is thrown away," <-- I imagine this is some form of moral in a story; Believing something like the skyline belongs to you is foolish and arrogant, as such, the fool will be done with. "Our hooves are tangled in the barbed wire," <-- Here it becomes possible that the story is told by a horse, or Centaurs even. "I know that these thoughts will go away in time." <-- Implies either the story or the metaphors do not actually exist, and it's just someone overthinking due to human weakness.