Except it really confuses about flat felled, mock flat felled, french seams
First, they have what they call a "mock french seam" which I would just call a flat felled seam - sew row of stitches, open fabric, fold over seam allowance twice to hide raw edges, sew another row of stitches.
Then they have what they call a "French (or Flat Fell) Seam" which I would also just call a flat felled seam - same as above except the seam allowance for one fabric is half so the seam has one fewer layers of fabric. If you have thicker fabric or you're overlapping seams, having one fewer layers of fabric makes it easier to sew through.
And they don't have a French Seam - sew two pieces together with a row of stitches, turn inside out and sew another row of stitches creating a fold hiding the raw edges. This seam is probably not useful for kite sewing but useful for clothes.