Amazonian here! (Speaking for myself, not for the company.) You can use any runtime you like for your Lambda functions, even if it's not supported as a first-class native runtime. The simplest way is to build a container image that has the runtime you want to use. All you need to do is to include the Lambda Runtime Interface Client with your image, and set the proper ENTRYPOINT.
Doesn't using a container image hurt the Lambda experience a lot? Longer cold start times, needing to maintain the userland dependencies (and not just the language-level dependencies) in the container image?
See https://aws.amazon.com/blogs/aws/new-for-aws-lambda-containe... for a blog post, and https://docs.aws.amazon.com/lambda/latest/dg/images-create.h... for the reference documentation.
Once you get the hang of it, it's super easy, and you may never want to go back to the old way!