It isn't a lot of money for industry research. Changes of +-40% in productivity are an enormous advantage/disadvantage for a large tech company moving tens of billions of dollars a year in cashflow through a pipeline that their software engineers built.

No idea. They don't say who they were; just random popular GitHub projects.

To be clear it wasn't $75k each.

You can see a list of repositories with participating developers in the appendix! Section G.7.

Paper is here: https://metr.org/Early_2025_AI_Experienced_OS_Devs_Study.pdf