The blue and yellow block posted above doesn't look like it should be called Lemoyne Star due to fabric placement. There is different contrast shown in other pictures for the Lemoyne block.
To sew Y seams, just stop sewing 1/4 inch before the intersection of the next fabric. Lock the stitch or back stitch to hold the seam.
There is a video here for the die. The pieces and units are sewn together without Y seams.
https://www.accuquilt.com/go-lemoyne...-finished.html