Hey MadKnife
I have been living in Autofill land since it's been implemented (even before it was globalized) and can tell you with 100% certainty that autofill ignores both primary and secondary roles of the other players on your team. If you get designated as going to be autofilled this game, you will be assigned to a random position that is not one of the two you picked, and the rest of the team will be built around that. This means, for example, a top lane main can get autofilled into support/adc/whatever, while a top secondary can take top, or someone can even get autofilled top. It's really not uncommon for an adc player to get autofilled top and the top player to get autofilled to adc. It happens all the time and I bet if you could see the roles that people queued up with in the champion lobby, people would be like "WTF?" more times than not because of how little it makes sense.
With that said, I get my primary more often than not, and surprisingly only rarely get autofilled (queued as mid/jungle) but I'm not blind about how broken autofill is. It's all about achieving some quotas just like how the matchmaking system tries to force your winrate to be 50%. It tries to autofill just for the sake of autofilling, and doesn't respect the roles available on the team at all.