What's difference between node_revision__field_X and node__field_X tables in Drupal 8?
Yes, that is correct. This is the same data structure as in Drupal 7.

It would be possible to write a different backend that only uses one table, there was a project for that for Drupal 7 but I think it never really took off.

See https://www.drupal.org/node/2083451 for a discussion if that's still necessary or not.