I think being the best at something, not only tf2 comes down to to how competitive the thing is. There are people that are naturally talented for some activities and we all know that, but the person still needs to develop his natural talent through practice. Of course the ammount of work he gotta put in to reach a certain level will be considerably smaller than an ordinary person, but he still needs practice. What we dont know is how big the difference is, and that's what make things hard, because at the same time you are putting all your hard work into something there might be someone who is willing to do the same and it will require 2x less work than you to reach the same, that's why being THE BEST at something is really hard.
If we were talking about some popular traditional sport, no you cant be the best just by pure hardwork as naturally talented people from all around the globe ended up finding out that they are naturally good at the sport due to its popularity/exposure, and they are practicing as hard as they can to become the best at it. But we are talking about tf2 a small-sized esport title. The competition isnt too big. No one here except b4nny makes a living off the game, we've all got irl things to do, so we end up treating the game as entretainment/a more serious hobby. So the chances of naturally talented people grouping and grinding until they become the best are actually much lower when compared to bigger esport titles. So yeah i think its possible for someone to become the best at tf2 by working really hard.
I think b4nny is the perfect example to ilustrate that. We know a few of other invite/prem players that can be considered more naturally talented than b4nny, but as he put much more hard work into the game than anyone else here he still manages to be the best. We dont know how "big" b4nny's talent to the game is as there isnt any exact way to measure it, but im pretty sure people here can cite some examples of players they think are more naturally talented and also are extremely dedicated, but b4nny with his 15,000+ hours into the game still at the top.
Also, this hard work vs talent dichotomy isn't so accurate. Multiple factors can influence someone at becoming the best at something, not only those 2 factors. Environment, opportunities, mentality, health and other factors are also important.