pretty sure implies he has a hunch that they did, not 100% certainty. They may've said they hadn't considered such a thing before in a "forum" that the OP was able to overhear this either directly or indirectly and it wouldn't be a stretch to consider that in Facebook's history of borrowing from others, they then borrowed it from reddit.
It doesn't discount that it existed prior to reddit, just that they may have been the source for Facebook.
Microsoft was selling Office 365 subscriptions for a while and I never bought one. Eventually, I bought one from Amazon. So it is both true that I got it from Amazon, but that Microsoft had them prior.