I know of several sites/people which insist on correcting this phenomenon through a couple strength exercise techniques and focus on biceps femoris muscles. They never showed what to do for pigeon toed, bow legged, or knocked kneed athletes though. Thus I was instantly skeptical as if you can solve the duck feet problem then you can solve the others as well.
Biceps femoris is a main factor in foot splay/external rotation about as often as Adenotrex is a main factor in dunking(https://elitetrack.com/blogs-details-4048/). Just because Louie Simmons says it doesn’t make it the cause.
Also consider sartorius which when tight can cause the leg to suddenly rotate outward as soon as it lengthens to tension near BDC. If anyone knows a good sartorius mobilizer, let me know because that is a significant issue for me personally. I usually resort to trying to pull the muscle off my ASIS as hard as I can with my thumb which does the job but isn’t exactly easy and elegant.