Aren't there abstractions to reduce the impedance mismatch between serverless offerings, e.g. Serverless (javascript framework)[1], which should allow easier portability to self-hosted solutions - openfaas or openwhisk etc - including running in containers on more traditional infrastructure, which is cheaper for this particular scale and use-case?

Sure, they're still FaaS which seems to be the unit of deployment for the serverless movement. For (hidden) managed server container deployment, Fargate is the offered solution I believe.

[1] https://serverless.com