While true, it's not the whole story. lambda is also approximately 1000x more expensive than EC2. So if your lambda is only running 0.1% of the time or less (12 seconds per every 3 hours 20 minutes) you break even. If it runs more than that, lambda is more expensive.

This is just price comparison. There are other reasons, convenience, visibility, integration with other services, to choose lambda sometimes and EC2 other times.

It's important to profile and know what your use case is!