So I emailed the lead (only?) developer and got a pretty quick reply! It turns out he adds free Steam keys into the game every time he does a major patch. Below is a copy of his reply to me:
Keys to Hacknet can be found in a few places ingame. They're the same for everyone, but I change them on major patches (and some minor ones) for new saves created after that patch - first ones in get them. I also scatter in some used keys and traceable keys to stop people taking them en mass.
I've changed the type of key I generate for this after using my old beta keys for it gave me some trouble on some accounts.
Basically what I understand is that after a patch he adds in some free keys and the first people to find them win a free copy.
According to one user it appears that at least one of the keys listed is your own.
it is your Hacknet's steam key
I'm not able to confirm that directly as I bought my copy directly through Steam and did not see a key.
There is a Reddit thread, as mentioned by @ardaozkal, which seems to suggest the keys are from the beta testing.
Both come up as a beta key for hacknet but it's already active on a steam account.
Again I can't find any official information on this but I will keep digging.