The only relevant consideration for an EHC needs assessment is whether the child or young person has special educational needs and may need an EHCP.
Children and young people, for example with High Functioning Autism (HFA) are likely to have special educational needs. Children with HFA will demonstrate social interaction difficulties, social communication difficulties and rigid thinking.
If your local authority refuses to make an EHC needs assessment purely on the basis of academic achievement, that is failing to address the law properly.
Any blanket policy that states only people who have low academic attainment have a ‘learning disability’ is irrelevant since a person may be ‘disabled’ even if they have a high IQ. In law, a ‘disabled child’ includes people with high-functioning autism, ADHD and personality disorders.