No, this does not meet the goal. Assigning a built-in policy definition to the subscription is not enough to ensure that when an NSG is created, it automatically blocks TCP port 8080 between the virtual networks. This is because there is no built-in policy definition that matches this requirement. The closest built-in policy definition is "Network security groups should not allow unrestricted inbound traffic on well-known ports", but this policy only blocks TCP port 80 and 443, not 80801.
To meet the goal, you need to create a custom policy definition that enforces a default security rule for NSGs. A policy definition is a set of rules and actions that Azure performs when evaluating your resources2. You can use a policy definition to specify the required properties and values for NSGs, such as the direction, protocol, source, destination, and port of the security rule. You can then assign the policy definition to the subscription scope, so that it applies to all the resource groups and virtual networks in the subscription.