I’d argue that all three of them would still be considered the co-founders considering how early Swartz was brought into the picture. Of the three though he for sure had the smallest impact on what reddit became after its first corporate buyout.
Yeah he is one of the founders, but Huffman was always in it more for the profit. Aaron Swartz was the one with the vision, he was an activist and believed in free information. Sadly Swartz committed suicide after being arrested for sharing academic journals from a secured computer at MIT. Federal prosecution decided to make an example of him for multiple felonies.
Same thing happens all the time in places with “edgy” or just plain offensive humor. Some people see it and think they are just being funny, others see it and think they found a group where hateful views are accepted. 4chan started the same way before it became a cesspit. Turns out when everyone is repeating racist and sexist jokes, that some of them aren’t joking.