Read through the post, it is explained there:

> The reason I started on this was to better interoperate with "site-to-site" vpns, in particular AWS Site-to-Site VPNs, and the Auto-Discovery VPN (ADVPN) stuff on fortinet fortigate appliances. Both of these negotiate IPsec tunnels that can carry any traffic at the IPsec level, but use BGP and routes to direct traffic into those tunnels.