In a true merger, you can't have vesting for the people from just one of the incoming companies, as you might if you were hiring someone or doing an acquisition. The reddits would have had to make their own stock vest in order to make Aaron's.

It surprises me that the co-founders of reddit did not have a vesting arrangement in place in case one of them decided to drop out halfway for whatever reason well beyond their control. It's good that things worked out well for them.

Did Aaron bring anything to the merger other than his future effort -- like some code that was ready to drop into reddit?

What universal business maxim dictates that? As long as everyone agrees to the conditions of the contract before being signed.

Can I ask why they would have had to do that?

you called out pg. the universe is now collapsing.