View serializable schedule or not
I assume that X refers to the same table row everywhere.

In that case, the two transactions in your picture are not serializable, and indeed a lost update would occur. Any serial execution of the transactions would produce a different result from the nested execution you are showing.

To clarify the term "serializable", here is how the SQL standard defines it:


  The execution of concurrent SQL-transactions at isolation level SERIALIZABLE is guaranteed to be serializable.
  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.


Wikipedia has this definition for view-serializable:


  View-serializability of a schedule is defined by equivalence to a serial schedule (no overlapping transactions) with the same transactions, such that respective transactions in the two schedules read and write the same data values ("view" the same data values).


That seems mostly equivalent, except that the SQL standard only talks about the effects, while Wikipedia talks about reading values as well. But if you read a value and that has no effect, you might as well not read it, so the distinction is mostly philosophical.

And the schedule in your question satisfies neither of these definitions.