That's why I think MongoDB's new license which prevents AWS etc from hosting as a service without open sourcing makes sense (https://news.ycombinator.com/item?id=18229452). Amazon barely contributes anything back to the community and is known for destroying OSS businesses.
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.