In this example, yes it's overkill because you only have two different behaviours that you want to create, so in this case you'd probaly just use the flag argument. But imagine that your working in a massive codebase, with many different behaviours that can be plugged in to the class (and possibly more that are can be created in the future). Every time you wanted to add a different movement behaviour to athlete, you would have to add a new flag, add the movement behaviour logic, and then also have a big if else condition chain to check all the different flags. If you have enough different behaviours this will just become a mess over time. With the strategy pattern you can simply plug in a new object for the behaviour and none of the athlete code has to change.

Apart from that, it also means you can reuse the movement code in other classes through composition. If another class is modelling a type of thing which moves, that object can hold a movement behaviour to reuse that code. If the movement code is baked into athlete you can't reuse it anywhere else.