Are learning disorders in a kid a sign of prior child abuse?

No. Learning disorders are caused by many things. But, child abuse, for the most part, is not one of them.
Not so simple. Common causes are heredity and prenatal or perinatal complications. However, child abuse deending on type and severity can impact the developing brain and lead to neurodevelopmental problems especially in cases of severe abuse or neglect. These can be complicated by head injuries, "shaken baby syndrome" malnutrition, etc. But one can't assume abuse if there is learning disorder.
Probably not. Learning disorders are almost never directly the result of prior child abuse.