Ruby on Jets is very opinionated on how it works with AWS services similar to AWS Amplify.
Ruby on Jets is tightly coupled with very specific configurations for AWS services and this could lead to less technical flexibility which could be problematic to adopt emerging service features or if you need to migrate part or in whole your application in the future.
Ruby on Jets has solved most common technical use cases and will provision and configure the services you need like API Gateway, Lambdas, the deployment pipeline, and many more AWS services.
Lamby is just a wrapper so you can deploy Rails to a Lambda. Lamby has documentation on how to solve many technical use-cases for Rails but they are not Lambdy specific, it would be just how you would bolt-on serverless services to a Rails app.
Ruby on Jets is how I would have thought a Rails-like app would be built on serverless architecture where each controller action is a Lambda, bu Lambdy being so simple and flexible, and showing you how to solve the most common technical use cases I would not adopt Ruby on Jets today.