It isn't my quilt either, but I'm glad you showed it to us. This is how I think it was made.
1. Cut a square of plain fabric A and patterned fabric B.
2. Sew a border of fabric B around square A - but stop sewing when you reach the corners. Let the tails of the borders just lay there overlapping each other. This forms block AB.
3. In the same fashion use fabric A to border square B forming block BA.
4. Lay block AB on top of block BA and draw a diagonal line, then sew 1/4" on either side of the diagonal line, forming 2 bordered HST blocks.
This way there are no miters needed for the blocks. (If you had sewn miters, you would just be slicing right through them when you made the HST's, so why bother?)
I think this would work, but I haven't tried it - yet - but I think I will. If you try it first, please let us know how it turns out.