ALL true lye-based soap will create soap scum if you use it in hard water. You can't get away from that fact; you can only manage it.
Dr Bronner is highly soluble in water because it's high in coconut oil and it is also a potassium-based liquid soap (at least the Dr B I'm familiar with). This type of soap will still form some soap scum, but is not as likely to cause plumbing problems as, say, a soap made with lard or palm. Not sure who has decided exactly the opposite, but that advice doesn't make sense
If you're wanting to use true lye-based soap to wash clothes, you want to use soap that is highly soluble and has a low superfat. In addition to that, you must use a water conditioner, such as washing soda to maintain a high pH in the wash water and also to remove hard water minerals. If you try soap alone, you'll be sorely disappointed.
I have no idea how this Mama Suds product is formulated or what it's supposed to be used for. Dr B isn't formulated for using in a clothes washer. Mama Suds may be okay for laundry but probably isn't suitable for bathing. Like I said before, all soap makes soap scum in hard water -- you can only manage and minimize the scum, you can't prevent it.