I do not see how there is an illegality.
it is illegal for parents to loan money to a campaign, if the funds are repaid? only the candidate can loan money?
I think that Guinta reported the money as having been donated by himself to his campaign, and the finding is that, because it came from his parents’ bank account, he should have reported it as a donation from his parents (which would have exceeded the limits per contributor). It may be that uncollateralized loans to campaigns are treated the same as donations.
I hate the goo-goos. Calling for a resignation over a violation of an unconstitutional campaign-finance law? Give me a break!