YC has nothing to gain by selling ads on this forum. The business is already extremely wealthy and successful. Any ad revenue from this would be absolutely peanuts. It just doesn't add up.
But let's be candid, It was a promotional piece as well and if I "owned" this forum I would certainly have something to gain by charging a modest fee for such placements. It may sound like I'm angry about this or have a negative feeling, but I don't; not at all.