It's one thing if you have a source of revenue so justify your S3 costs. My interpretation of the parent commentator's concern is that this person has opened up a multi gigabyte S3 file publicly and sent lots of traffic its way (via hacker news) for what appears to be a passion project.

This is a risk any time you host anything on the internet on any system that charges you for storage, bandwidth or CPU usage. It's fine to take that risk on as long as you're aware of it and keep an eye on your spend.

In this case I'm nervous because 43GB is a lot larger than most projects!

I think the point is that not every host charges for bandwidth or CPU usage, making S3 particularly risky.

There's no CPU charge, only per-request charges.

Bandwidth is free into S3 or from S3 to another AWS service so it's particularly handy for that use case.

For S3, yes.

Every system has risks. You just need to be aware of them going in.

S3 is excellent at many things, serving a ton of public traffic isn't one of them. Usually people put a CDN in front of it for that use case.