Amazon just open sourced the hypervisor used for Lambda and Fargate ( https://github.com/firecracker-microvm/firecracker )

When the Spectre/Meltdown hit, Amazon also open sourced the mitigation used for the AWS version of Xen ( http://xenbits.xen.org/xsa/xsa254/README.vixen )

If you search the Linux git commits for people with @amazon emails, you'll find contributions, bug fixes, etc.

I don't think it's fair to say that barely anything is contributed back.