This anecdote is a lot less interesting, both because of the separation (you know some people vs they run a company with direct exposure) and lack of detail. I'm sure you do know some people who contribute, but you haven't given any details about their experience that would contradict OP's claim that contributing is hard.
I work at AWS in Professional Services until tomorrow. I worked with the SA and had meetings with the service team responsible for the AWS Service in question to discuss the API shortcomings that we needed for automations. He contributed to Terraform once the APIs became available from our service team and I wrote the equivalent CloudFormation custom resources for a project (and open sourced on the public AWS Samples GitHub repo after going through the approval process) as part of a larger project.
I found a bug in the underlying service API that affected both my implementation and the Terraform provider my coworker wrote. I posted a sample in our internal Slack channel where the developers of the AWS service hang out and they fixed the API bug relatively quickly.
My coworker, had no problem getting his TF code merged in. I know the code he wrote and I went to the GitHub page before posting this and it is in there.
Later as native CloudFormation support was added, I replaced my custom resources with the native equivalent as part of the larger project I was working on.
There's probably not much AWS contribution to the core of Terraform from AWS, but there's very little contribution to that from anybody outside of HashiCorp because contributions happen on the providers.
AWS is definitely involved with their provider though. AWS ProServ built out a whole account vending machine thing that was in Terraform (the name escapes me atm), and various other service teams and SAs are regularly involved in contributing to and growing the Terraform ecosystem.
It would be super disingenuous to imply AWS is not contributing to the success and growth of Terraform.
