Guacamole Sitting out?
Nobody is telling you "or you'll get sick"

What they are telling you that it's unsafe. This is a completely different thing. You can eat unsafe food if you like. The people who publish unsafe recipes do it too. They just count on not getting sick. 

2 hours vs 4 hours 

The actual "danger zone" time is 4 hours. For foods you buy in their final, perishable state, it's down to 2 hours, because it's assumed that the first two hours got "used up" between producer, transport and seller. So it just so happens that 4 hours for a homemade guacamole (whose clock started ticking when you cut the avocado) is still safe. But this is a coincidence, you probably can find recipes somewhere which say to keep it out longer than 4 hours before serving. 

let it sit out for 2 then serve it (and it sits out for 2 hours) 

That's not what a recipe assumes. It assumes that you let it sit for 2 hours and then serve it and it gets eaten. The recipe creator assumes that you know enough about food safety to make your own decisions if your process is different.