The serverless.com framework is built around existing serverless/faas platforms and allows you to write vendor-agnostic functions and deploy them exchangeably. It does not provide the resources to run your code. it abstracts away different API of different vendors to make them seem as one.

Knative on the other hand provides building blocks to eventually build a serverless platform. It provides the resources necessary to run and scale your application.

In short: You could use the serverless.com framework to deploy applications/functions on knative. But you still need some layer actually running your workload, like knative.

not sure they seem quite similar (I am not familiar with neither to be honest) but the main difference in my opinion is this is built on open source and backed by the big boys : google, IBM, SAP ..etc