Okay. So I think we have all been wrong.
The thing that has been bugging me is that we aren't adding all of the endpoints for the ALB. The module, this method, the existing method in existing dns module, all use the same "structure" for the same "end_state", that is a CNAME pointing to the ALB.
However, this is AWS. For an ALB in AWS, Route53 has special functionality, an ALIAS record.

1. This is NOT the same thing as a CNAME.
2. The special functionality is for the specific use case of an ALB/ELB needing a single endpoint.
3. there is no ALIAS function in the upstream terraform-modules/aws-dns, which leads me to believe we haven't used this.

resource "aws_route53_record" "www" {
  zone_id = aws_route53_zone.primary.zone_id
  name    = "example.com"
  type    = "A"

  alias {
    name                   = aws_elb.main.dns_name
    zone_id                = aws_elb.main.zone_id
    evaluate_target_health = true
  }
}

The difference, this allows a single record to point to all the records of the ALB.
I am looking to PR the functionality into aws-dns, but we can use this module to figure it out in the meantime. Of course, I'm writing this before testing it, so I could be wrong....

I have implemented the described approach on the lb_cname branch and will open PR#8.

Closing in favor of using the alias record.