It's based on media reports like https://electrek.co/2016/11/30/gm-chevy-bolt-ev-loss-before-... and http://gmauthority.com/blog/2017/06/ubs-gm-probably-loses-ab....
Electrek report is based on taking to GM insiders. gmauthority is based on a vehicle teardown.
For what it's worth during earnings calls Tesla discloses profit margins for their cars.
As I understand it, GM never publicly disclosed profit margin for Bolt so the unofficial guesses is all we have to go on.
It paints a hopeful picture if you're interested in the future of electric cars, I think. The Bolt is something GM threw together with the minimal amount of effort necessary, understanding that this generation would have pretty low volume, and it's honestly really good for what it is.
An additional 72 mile range for the Model 3. And a standard compact car form factor to Chevy’s sub sub-compact. I’m sure that yields some shoulder room and other comfort benefits.
The form factor is also not a technical thing, it's the sort of thing I was trying to head off with they aren't really aimed at the same market segments. I think it is fair to say that the Model 3 is the more desired car, but that's not the same as demonstrating a big lead in electric platforms.
Whatever Tesla is trying, they aren't actually delivering that, either. The model 3 may have curb appeal (the interior is another story) but it certainly isn't at a low price (the base price is above the median total—options and taxes included—transaction price of a US new car purchase.)
The Tesla obviously has better mind share at the moment, but various aspects of the vehicles are pretty comparable (even though they aren't really aimed at the same market segments):
https://www.consumerreports.org/tesla-model-3/tesla-model-3-...
I don't see a generational advantage for Tesla when I compare the 2. I see Tesla having a very short amount of time to match quality levels and production volume.