Is it true that a child that does not point must be autistic?

No, not true. A child who is growing fine, developing skills on time, interacting with other people, emotionally nice to be with, and smiles happily is likely normal. Such a child may point or not point, but it shouldn't matter, as the rest of his life looks normal. However, if a child has several odd traits or behaviors, and one of those behaviors is "not pointing", then he might (not must) be autistic.
Not always. When evaluating children for autism, there are a few symptoms that toddlers may or may not exhibit, pointing is a clue, making eye contact and observing the child for approval from parent when they do point, are all helpful when considering a diagnosis of autism. Talk with your child's doctor if you have concerns. Most of what I have discussed here are signs that are helpful at 18-24 month range.
A Sign. Lack of pointing in a toddler is a sign, taken with other symptoms of potential problems.But dr kwok's response is basically correct.