Put another way, how would you implement this without self-managing pieces like nginx/apache, rabbitmq/kafka, or mongodb on a compute instance?
The pieces of managed infra they're depending on are all pretty interchangeable from one cloud provider to another. It's not the cheapest way to solve the problem if you discount the cost of management, upgrades, etc. But if you factor those costs in, it's quite competitive.
Certainly, I wouldn't get any vendor-specific services like Lambda or SNS (in this example)
But still, when I first started in AWS we had lift and shift'ed most of our platform onto AWS and when it came to adding new features I first looked at Lambda instead of building out on the existing platform. The speed at which I could setup the feature and start measuring and iterating on it was so much better compared to doing it on our existing platform. Even though that was pretty ironed out and fully automated.