I doubt the traffic ever actually leaves AWS. Assuming it does make it all the way out to their edge routers, the destination ASN will still be one of their own. Not that the pricing will reflect this, of course.

The other problem with (interface) VPC endpoints is that they eat up IP addresses. Every service/region permutation needs a separate IP address drawn from your subnets. Immaterial if you're using IPv6, but can be quite limiting if you're using IPv4.