I worked there. Services running on GCP are a significant part of the internal service infra (ml platform, etc.) and it's not impossible that the abrupt loss of GCP would cause user-facing problems. The GCP spend was many, many times the AWS spend. Unless things changed since last November, AWS is not a meaningful part of the internal or user-facing infra.

With respect to DynamoDB specifically, Twitter has its own custom distributed key-value store: https://blog.twitter.com/engineering/en_us/a/2014/manhattan-... that twitter.com itself runs on.