You're right! I came across this issue when I tried to use my isPrime implementation in a Kata.

Good point! The thought crossed my mind while solving, but this passed the tests, so I kept it as-is.

IMO, casts are a red flag for bad design (that's also why the syntax is so ugly), so if you can get rid of one, that's a good idea. It has no runtime impact, indeed.
I am also not 100% certain about how it works, just that it makes code a little more readable as the functor is defined where it's used, instead of the reader having to look it up.
But there are really minor points.

That static cast to int is not required due to the C++ integer promition rules: every arithmetic operation is converted to at least integer size.
Also, have you considered using a lambda instead of the acumCubes function? Worthwile.
Other than that: my compliments that you are using algorithms as much as possible! That makes this the first solution I've read that's actually easy to read and comprehend.