Is it even or not? Isn't what you are exactly asked for?

Good point! I shouldn't blindly trust tests....
I completely agree that code readability is very important. However, analyzing the struggle at the kumite, I noticed that "shorter is better", this criterion guided me. Thanks for your comment.

This code says that division by zero is possible:

isDivisible (5, 0) // returns true.

This is because 5 % 0 returns NaN and !NaN is truthy value.
More predictable code:

const isDivisible = (a,n) => a % n === 0
isDivisible (5, 0) // returns false.

Also note whitespaces around %. Google Style Guide recomends this notation for the sake of readability.

Yes it is in javascript, UpperCamelCase could be used for classes

Isn't lowerCamelCase more common style to name functions?