It isn't a myth, I have three quilts that prove it can happen.
Just washing two sewed pieces of fabric 50 or 500 times will never cause the thread to cut the fabric, it is the weight of the three layers shifting over time and washes and wear that create the cut.
Some reasons the perfect storm can converge are:
using worn clothing for the patches
flat fold thin fabric for the patches
quilting too far apart for the batting selected.
It probably won't come up if the fabric is good quality and I think we all agree price isn't the only factor in fabric.
If the quilting stitch is mid-range rather than too tight
Consistent full seam allowances.
All that said I've been making quilts since 1970. Some of those early quilts were just like beginners quilts today.
The difference is, 1970 was a long time ago and my quilts are used and washed often
so I've had time to see what happens
any quilt folded in the closet or under the bed will probably last forever, regardless of materials, skill and use.
Cotton thread will let go before cotton fabric does. Those problems are super easy to fix. patches cut on the sewing line are a booger to repair.
That said, I am happily using up all my coats and clark poly cotton thread because the quilts I'm making today won't have to hold up to lots of everyday wear because I have a lot of quilt and switch them out every month.
Quilt police is the myth I don't believe. The only quilt police are in our head.