Yes, we did have to implement our own state tracking, retries/recovery, etc- but since we were focused on a limited subset of the cloud API, this was pretty easy.
So you re-implemented terraform but worse most likely.
Also you could have added those missing features and re-use the TF engine, it's very simple to include new API of an existing provider.
No, we used an entirely different architecture/paradigm not possible with tf, and had capabilities tf doesn't attempt to provide (such as coordinating migrations with both cloud and application APIs, or managing capacity while upgrading 10000s of CPU cores worth of compute).
