It is both.

I used to think the Great Man of History idea was silly. Clearly, it was a about a complex set of conditions that made somebody like them inevitable.

Then I worked for a few CEOs, good and bad and realized that, even in a company of thousands, the vision and charisma of one man makes all the difference.

Now, I think I see it like great plays in sports. It is the whole team, and the other team, and an uncountable number of factors ( bump in the turf ) that setup the conditions for that perfect shot. Once conditions are right though, you also need somebody capable of capitalizing on that moment and making history. Perhaps the most important element is that history making player. You still have to put them in the right spot though and a lot goes into that.

True, individuals aren't completely without influence of course, it would certainly re-roll details of what exactly happens, which could turn out significantly better. Unfortunately it could also be worse.

Also, there are limits to what can be achieved with it. For example, even without Hitler, to achieve a successful socialist movement in Weimar Germany, you'd have to do a lot more, Hitler was raised into a fascist demagogue by organizations active long before him, and their foundation was so rooted in the sense of revanchism and the rampant conspiracy theories drenching with antisemitism in Germany at that time, I genuinely think he was not important enough as a rallying figure to have tipped the scale, it was weighted heavily in their favour to begin with.

Keeping the SPD from hiring and strengthening the protofascist Freikorps alone would be more influential than killing Hitler, for example.

Indeed. Some people wrong place wrong time completely alter the course of history from that point forward.