These are fairly different offerings. Improbable is about enabling the creation of massive worlds with various actors and players interacting in the same world server - effectively large-scale MMO's with a lack of sharding. But it is also proprietary technology, so your game is entirely dependent on them sticking around and maintaining the software.

Agones is focused more on massive scaling for individual servers. i.e. making it significantly easier to scale up and down servers based on demand (similar to AWS GameLift), but from what I see there is no world-sharing across them as that isn't the point of this specific project.

It's an interesting solution to a common problem. We work in a similar space and use Kubernetes extensively for managing our services behind the scenes, it's particularly useful for mobile games as they have a lot of peak/trough traffic throughout the course of a day, let alone during a featuring on an App Store or a live event which drives traffic through the roof for a very brief period of time.

This looks more like a competitor to AWS GameLift, infrastructure designed for hosting game servers which are different than your typical web server (almost always stateful versus stateless). Improbable is more a developer level SDK to drop in your game and wire up functionality to their 'cloud processing'.