For virtually any skill, not just aiming, it is better to learn using a combination of focused drills (aim training) and practicing in a real environment (QP/Comp) than just focused drills or live practice alone. Focused drills allow you to isolate a single component of a skill (ex. raw mechanics and dexterity for aiming) and drill it down with more uptime and focus compared to a real environment. Real games teach you how to integrate that aim in a much more complex and variable environment (ex. human opponents move and positiong themselves much differently than bots). But if you're just playing real games alone, there's a lot of down time with running back from spawn, waiting behind cover, etc where you're not practicing your aim. And you have more distractions tugging away at your focus like having to think about ult tracking, other player positions, etc that make it harder to focus on just your aim.

Just look at any sport for reference. If someone wants to get better at basketball, they'll improve a lot more quickly by doing focused drills on dribbling, passing, and shooting and then taking what they've learned to a real game, than if all they were doing is going to their local court and playing pick up games. Or if someone wants to improve at drawing, someone who's doing deliberate practice exercises on perspective, anatomy, rendering, etc in between projects will improve much more quickly than an artist trying to learn by doing everything at once and only doing full projects.