I cannot grasp the definition of serializable
Which "same result" do we have in the example? Which reordering can we have to produce the same result?


The opening quote is an incorrect statement of serializable isolation.

The definition of the serializable isolation level in the SQL-92 standard contains the following text (emphasis mine):


  A serializable execution is defined to be an execution of the operations of concurrently executing SQL-transactions that produces the same effect as some serial execution of those same SQL-transactions. A serial execution is one in which each SQL-transaction executes to completion before the next SQL-transaction begins.


The example given is consistent with that definition.

The text above is taken from my article The Serializable Isolation Level