"... well, NLB runs on Amazon VPC. On VPC we encapsulate, authenticate and secure traffic at the packet level. Packets can't be spoofed or MITMd on VPC. Traffic only goes where you send it. That makes it possible to use a self-signed, or even expired, certificate on your end."
https://twitter.com/colmmacc/status/1088516445145784320
"... well, NLB runs on Amazon VPC. On VPC we encapsulate, authenticate and secure traffic at the packet level. Packets can't be spoofed or MITMd on VPC. Traffic only goes where you send it. That makes it possible to use a self-signed, or even expired, certificate on your end."