Why is an assignment an expression without giving any value ?
It does give a value: the value of the just assigned variable. E. g.
v = x = c;
is equivalent to
x = c; v = x;
as the value of
x = c is the value of
x after the expression was evaluated.
The explanation is correct.
But you can not rely on the order of side effects in an expression.
Therefore, it might in general not be the case that a nested assignment behaves the same as a sequential one.
Especially, if more than one side effect are performed in an expression.
For instance with
arr[arr[x]] = arr[x] = y or even simpler
x=y=y++ with nested assignments).