> OOP won against functional because OOP is easier to understand and leads to more maintainable code.If by functional you mean higher order as in the title and not what is commonly known as FP - it's never been true. For example, a function that has no outside side effect and that calls an anonymous function provided as an argument is way easier to understand, reason about and maintain than its OOP equivalent of creating an object and calling three methods in specific order mixed with your code.
> These days people are taking a second look at functional because functional avoids state, which makes it easier to write concurrent code with no locks.
Avoiding state doesn't make it easier to concurrently touch shared memory without locks. Event loop style concurrency simply doesn't touch shared memory concurrently, so it doesn't need locks and doesn't have all those multithreading problems. It can still mutate state left and right, higher order programming just allows to map intent better to the code, making "sequences look sequential" and all that.
Actually solving problems concurrently with high performance today generally requires ditching shared memory based synchronously communicating concurrency models, leaving async communications of isolated entities where locking problems and overhead don't exist. But it has nothing to do with functional programming, OOP, higher order programming.