The outgoing owner of the Warriors opted for the lower offer to keep the team from moving to San Jose, it was not a forced move by the NBA. If it was, it's not public knowledge nor was it done officially, which is how the NBA gets people to do things they legally can't, scare tactics, not legal arguments.
And agreements to not sue are null and void in antitrust cases pretty much all of the time. If an entity is monopolizing an industry (professional basketball in this case) and you want to get into that industry, you have no choice but to agree to whatever conditions put out there by the monopolistic force because that is the ONLY gate of entry, essentially. By nature, most of these agreements tend to be under some semblance of coercion for that reason and rarely hold up. The only exemptions from antitrust litigation are granted by the government, not permitted by an entity in violation of antitrust laws through their own private bylaws.