"Being attentive to your practice (i.e. recording, going over your work, etc.)"
If you practice the same thing over and over, you won't get better. If you fail, figure out what you did wrong, and improve, that's competition.
"Asking, and taking the advice of other people in your field"
I will agree with you here.
"teams more diverse in culture, race, and gender,
'diversity of thought' has nothing to do with race, gender, or culture. I've found that many companies will use inferior ideas just to say that they are 'diverse'.
You also have to be careful, because when you take too many ideas from people that lack experience/expertise, you have to tune out the noise.
I do agree you need to get a wide array of ideas, though, regardless of race, culture, or gender.
"in a game jam session, you're collaborating with those around you to produce lots of interesting and good art."
This isn't competition, and there is a place for it..but this isn't really what we are discussing.
"another perspective, a second pair of eyes, the well-worn advice of the 40yro burned out techies"
Most learning like this happens if you get stuck on something and don't want to spend lots of time on it (although failing until you succeed will allow you to learn 5X more).
However, to take what you learned and actually improve, takes competition.