The problem is that being "flatmappable" very strongly implies that it's a container, but it doesn't have to be. It especially privileges the list implementation over other things. It's much the same as saying "Iterator may be a bit difficult to handle, but it's basically a way of presenting the elements of an array one at a time"; it isn't that that statement is wrong, it's that it is incomplete, and really leads people to thinking that iterators can only iterate on things you concretely have in memory, when instead they can iterate on anything (integers in sequence being popular), and there's also an "advanced mode" where you can drive the iterator (frequent example: an iterator to walk over files on the disk that can be fed back to to not descend into a particular directory upon inspection) that you'll never understand if you see iterators as "a way of walking an array".

You can understand flatMap and still not really understand "Monad", just as you can understand array iteration and still not necessarily understand "Iterator".