Because you're not used to abstract algebra. JavaScript arrays form a monad with flatmap as the join operator. There are multiple ways to make a monad with list like structures.

And you are correct. Monads have nothing to do with sequencing (I mean any more than any other non commutative operator -- remember x^2 is not the same as 2^x)

Haskell handles sequencing by reducing to weak head normal form which is controlled by case matching. There is no connection to monads in general. The IO monad uses case matching in its implementation of flatmap to achieve a sensible ordering.

As for JavaScript flat map, a.flatMap(b).flatMap(c) is the same as a.flatMap(function (x) { return b(x).flatMap(c);}).

This is the same as promises: a.then(b).then(c) is the same as a.then(function (x) { return b(x).then(c)}).

Literally everything for which this is true forms a monad and the monad laws apply.