When you have text messages from Bill Belicheat 3 days before your job interview with him mistakenly congratulating you (Brian Flores) for obtaining a job that eventually went to a different Brian (Brian Daboll), and which includes Bill Belicheat apologizing and acknowledging he sent it to the wrong Brian, it becomes much, much easier to establish a timeline of events.
Then consider the proportionality determination that occurs throughout the broad-ranging discovery process in federal court generally, and combine it with the fact that a federal court judge in the SDNY has already been provided (on the face of the Class-Action Complaint) evidence to suggest the interview itself was a sham. This will likely result in SEVERAL NFL executives and coaches having to turn over e-mails, text messages, and telephone records related to Flores's claims and the NYG HC job (and we have already learned from John Gruden how flippantly these guys communicate with one another in writing) during discovery. And should Flores's attorneys find additional evidence to corroborate the allegations in his Complaint during that process (e.g., communications from NYG executives/front office that Daboll would be hired, prior to Flores's interview), it will become a lot easier to prove by the preponderance of the evidence that the NYG participated in a discriminatory hiring based on race.
To state a claim for discrimination in hiring, a HC essentially needs to show he is a member of a protected class (here, race) who applied for and was qualified for a HC position, was rejected, and the position was filled by a non-minority candidate (this can vary depending on state law claims vs. Section 1981 claims vs. Title VII claims, but the nuances in the burden of proof aren't critical for our discussion at this point). To be clear, Flores is not filing a lawsuit "for violation of the Rooney Rule." That would be asinine. Instead, Flores's lawsuit very clearly asserts violations of 42 USC Section 1981, NJ's Law Against Discrimination, NY's Human Rights Law, NYC's Human Rights Law, and states it will later be amended to include a count for violation of Title VII of the Civil Rights Act of 1964 (because there is a legal requirement that prior bringing a claim for violation of Title VII, a putative plaintiff must first bring a charge of discrimination before the EEOC and receive a determination on that charge, in what is typically called a "right to sue letter"). Flores (and his attorneys) are using the NYG and NFL's apparent disregard of the Rooney Rule (it's own rule, meant to curtail discriminatory hiring practices in the NFL, BTW) as evidence of an intent to discriminate on the basis of race in the hiring of HCs like Flores.
Whether Flores can prove his case or not remains to be seen. But that's the lawsuit, in a nutshell. A violation of two federal statutes, one state statute, and one city ordinance re: discrimination in employment.