That's the best I've been able to figure out as well!
I was at school with two Lyndalls, not related to each other or to the Schreiner family; one of them had parents who just liked the name in general, but the other started life as Ilse-Ann after her two grandmothers. Eight months later, her father (a lawyer) came home on a slack day when he'd been reding a cowboy novel at work to pass the time - and the heroine's name was Lyndall. He announced firmly that the baby's name had to be changed, and changed it was. We all felt that she'd been lucky.
Now, many years later (ah, me!), in the new, democratic South Africa we have a black Cabinet minister whose name is also Lyndall. Nice how you can't keep a good name down.