Yes indeed! Few people recognize the flag, but that’s what it is.
The flag also re-appeared briefly in the Spanish Civil War as the banner of the Carlists (aligned with Franco); in it’s original form (saltire of Burgundy) it had a “saw-toothed” design (not smooth bars). The smooth lines in those state flags are more akin to the “cross of St. Patrick” (which when added to the Union Jack gave it its current form). Before Ireland was involuntarily added to the “union”, the Union Jack had no red “X” in it - just the white “X” of Scotland’s St. Andrew’s Cross (until 1801).