Every programmer of course reads some source-code because they must read their own source-code. But such a programmer might use an Open Source library without reading its course. Whereas if you are truly hacker you are interested in how things work and you would more likely be reading such source.

I agree that definition of "hacker" is somewhat vague but mostly people understand it the same way depending on context.

I assume that reason dedicated programmers are called hackers is that earlier the the word "hack" referred to writers.

https://en.wikipedia.org/wiki/Hack_writer

What the hack, there's even a pub called "Old Hack"

https://www.tripadvisor.com/Restaurant_Review-g188644-d10512...