There's never an implication that they're amps. At least for the basic four Styles, anyway. The primary focus of them is merely methods to approach combat, such as focusing on projectile fire, or focusing on a more head on melee attack type fight. This also extends to Trickster and Royalguard, with the two merely being applications/abilities he already has and the game just limiting you to pick one "Style of play," at a time. Dante does have other things that amp those factors though, such as his reactive evolution, Quickheart, etc.
This does disregard specific abilities though. For example, while the Royal Guard style in itself traditionally doesn't "amp" him, in DMC4 the highest skill for it was a Juggernaut state that did exactly that. So there may be abilities within them that perform such a task, but the Styles themselves do not achieve that. They're just a manner of play to suit an individuals idea of how combat should function, whether this be evasion and quick movement, rushdown, zoning, or being a nigh-impregnable defense.
Or, to shorthand it, the Styles are abilities Dante already has and don't amp him in any way outside of specific abilities within the Style that can perform such.
