You have to make a choice at some level of the stack, for me, that was serverless as the deployment framework. You could probably use terraform or ansible + some custom scripts, but then you could argue you're locked into terraform or ansible ;)

The login thing isn't actually necessary (or if it is, that's new)

My most recent serverless configuration file was probably approximately 90 percent aws specific (cloudformation) syntax anyways.

Maybe the image is outdated? The Quick Start doesn't seem to talk about Serverless accounts.