What I don't understand is why we credit_genesis only if the payload author is const.GENESIS_ACCOUNT, in other words, if it's you who is reblogging.

I'm not sure why the JSON fields are named the way they are, but that particular "author" belongs to the identifier of the post that is being reblogged. So it's not meant to be the reblogger, but rather it's the author of the post that is reblogged. Check this reblog I just did: https://steemd.com/tx/7b23653de4a5ddfe49b7f247e4d1f9fb04ec9cc6

Email is best; I'll send you my email address on discord.