Should be manageable and it is probably less than you would imagine. Just checked real quick: the isos load from download.bazzite.gg, which is a Cloudflare IP. So they are either using it as CDN or even more likely use Cloudflares R2 storage for isos - which would mean they pay for storage (~15$/TB) and operations, but not for egress. This is seems ideal for few but huge files.

So for a single iso (~7 GB) they would pay 0,105$ for storage monthly and additionally 0,36$ per million of class B operations (reads/downloads). Of course they host more than one ISO, but for this example it would have been downloaded about ~150000 times to reach the petabyte.

So yeah, the ISO download is probably less of a problem. (Disclaimer: lot of assumptions, check in with a bazzite dev for clarity)