That hypothesis is stated in both the article and the source material. However, even if financial considerations are a part of it, they can't explain the full extent of the discrimination though - that 4H & Future Farmers of America count against an applicant while all other extracurriculars help.
This wasn't a double-blind study where for applications where all else was equal, 4H hurts applicants.
This was a statement about the populations. The population that is in 4H is less likely to get in (or likely to even apply) to top schools than the population that is in, say, debate. That's not really controversial, although it's worth theorizing about ways to improve the situation.
