Having a worker that does one request, processes that one request and returns a result isn't accurate, simple or safe? Scaling that simplistic interface in a 1:1 manner across many systems via ECS is pretty straight forward.

Now, in addition to probably optimizing what they've done, converting to, for example another container system, like K8s where they can scale vertically a bit better may have been another approach.

The biggest issue that I see is gRPC doesn't work that great with Node. You can't use it with cluster which means you have to self-manage threads/processes and it adds complexity there. Yeah, there's definitely issues that come into scaling in terms of performance optimization.... but where they started from isn't unreasonable imho.