So you are saying if I take my AWS infrastructure, which I launch with my terraform config, I can simply use that same config to launch the same infra on GCP or DO? Didn't think so. It's only slightly more agnostic than AWS CloudFormation, in that I can at least use some of the same conventions when I completely rewrite my terraform files.

I think you are confusing "agnostic" with "compatible with lots of different providers", but will require a total rewrite if you want to switch providers.