[iLoveOncall]

> I do wonder how they came to Lambda though

Probably misplaced hype about being fully serverless.

As you said, ECS would be much more suitable while also addressing the issue that they had (spikes in traffic).

Lambdas behind API Gateway sounds like a good idea and even looks like one if you don't have experience with ECS and don't do a proper cost comparison analysis.

My team uses ECS' autoscaling to scale up to tens (or even hundreds) of thousands of tasks in minutes without issues.