[GENERAL] Using xmin and xmax for optimistic locking

In the chapter "Using optimistic locking" of the book "PG Cookbook Second Edition" 

it is mentioned how the app can first fetch row from the table in the form
select a.*::text from table a where ...

Then do the work and then when it comes to committing do it as

   update table

          set ....

   where table.*::text = (saved from select).

If the row was changed between the time it was first read and updated, the

update will do touch any rows as the ::text will be different.

Why can't we use xmin and xmax columns to achieve the same.

select a.*,xmin,xmax into ... from table A

do your work

update table

    set ...

where pky = blahblah

and xmin = 2907587

and xmax = 0 ; 

I tested it and it works.  what I did was to select xmin and xmax and then sleep for a min.

In the meantime, I update the same row in another session.

After 1 min the update session failed to update any row because the combination of xmin 

and xmax was no longer true.

I was under the impression that xmin/xmax can not be used in the where clause for business logic as described above.  

Am I missing anything ?  If this works, it can make optimistic locking lot easier due to generic coding using xmin/xmax.