It's fairly well-established in research (and in practice) that there is no objective measure of developer productivity. Metrics like lines of code, number of pull requests, velocity points are incredibly poor proxies.
It stops being a good proxy if you use it to reward or punish developers, compare different languages, or different types of software. But if you don't do those, it's quite good.
Velocity points is worse, because the current culture implies it will be used to reward or punish developers. But it's probably still better than reported productivity.
In fact, reported productivity is probably one of the worst metrics around. People can't even keep track of how long they spend programing, much less of how much they accomplish.
Researchers who specialize in this field largely disagree with this. Here's a round-up: https://getdx.com/blog/measuring-developer-activity/
Oh, there is.
Money.
How much do you spend on eng to earn a buck (eng value, infrastructure). Impact of your next project on the bottom line. How much can you save by making a change.
So many bullshit features from engineers, and product people that have FUCK ALL impact on the bottom line. But hey they made you happy, or looked good on your resume to get you the next job...
So it's a bunch of surveys, not experimental data; AFAICT nothing here distinguishes ex. "devs who understand their code are productive" from "devs who are productive develop an understanding of the code". But forget correlation/causation, because it turns out their claims - even in the linked "research" is about how people feel, not their actual effectiveness - choice quotes are
> developers who report a high degree of understanding of their code feel 42% more productive
or
> Developers who have a significant amount of time carved out for deep work feel 50 percent more productive compared with those lacking in dedicated time
Notice that there's no attempt to objectively see if those feelings are true! SLoC might be poor, but you could look at literally any objective measure to proxy actual productivity and be better off than this!
So for all that I'd like to believe them, and all that the claims could actually be true, this is just a bunch of meaningless nonsense that barely qualifies as "data" and certainly shouldn't be taken seriously as "research".