I'm confused as to what you are asking - Jane St certainly hires plenty of people out of college not for nepotism reasons.
The fact that he did not have a prior job before college is not evidence he was hired for nepotism.
Let me guess - he also got into MIT through nepotism as well.
I'll let others decide on whether they find that reasoning convincing!
I know kids who definitely got in because of parents money, but it certainly did not impact their grading.
What grade inflation really means is that for many, if not all, majors at a school like Harvard, you can take a path of classes that will end up with you completing your major and having a high GPA.
That said, there are certainly classes that will be much less nice to you when grading and have a self-selected group of students. My guess is that if JS has enough people from, say, Harvard, they will know the difference between a student who took hard upper level courses to complete their major vs. just the simple basics.
In terms of why this grade inflation is so prevalent, one reason I think is that faculty want to have students in their courses/run a "superstar" course, and students select classes with the easiest grading policy. Schools like MIT (and Princeton as well) specifically combat this and so are known for grade deflation.
It seems entirely reasonable to me that Jane Street would consider a Bachelor's degree from MIT in math or computer science, being clever, and a prior internship at Jane Street as qualification to work at Jane Street.