The key distinction for me is that Lambda is event driven. If you have a microservice/function which is seldomly called, in Fargate you would pay 24x7 just to be listening for calls.
Fargate tasks can now be initiated by CloudWatch Events as well[1]. Lambda will always be at the extreme end of "serverless", but over time I expect the lines to blur more especially since Fargate will now be based on Firecraker[2] which has 125ms startup times for container workloads.