Calling it an alternative to SageMaker might be a bit misleading, as SageMaker is also a platform for training the models in automatically allocated EC2 resources, even on spot instances.
Cortex contributor here - you're right, I would say we can be compared to SageMaker model deployment. We are currently working on supporting spot instances for serving, and training is on our roadmap.
https://news.ycombinator.com/newsguidelines.html