Outside of AWS and the cloud bubble, bandwidth is extremely cheap. You can get dedicated servers (aka actual hardware) with 100Mbps of unmetered bandwidth for a couple bucks a month.
Oh.. Yes. I assumed that they hot linked (a word I haven't used in 10 years) all the images/videos from reddit / imgur. But no, they seem to re-host everything. Which is admirable both from a privacy and "fairness" perspective, it lessens the burden of Reddit.
But yes, without ads or fees, it seems unsustainable.
How much bandwidth was used by the virality of this post?
If this became popular I don't see how someone could afford to keep it going.