The FBI paid a company to find a few people's locations, and they were under the impression that the company would use in-house software, not NSO spyware.
At no point did the FBI use NSO spyware. Riva Networks did.
You can argue its misleading but I disagree. Outsourcing bad behavior to a third party doesn't remove culpability on your part. This is all to common these days and allows both parties to point fingers at each other while nobody faces any responsibility for breaking the law, violating people's rights, and/or unethical behavior.
The FBI just using assumptions and impressions on methodologies used by the company they contracted with is no excuse when they could have asked how the work was going to be performed. Incompetence is no excuse either.
iOS now has a "Lockdown Mode" which is supposed to be more secure against pegasus, but can break some functionality, but then again, it's a closed source OS so you'll have to take their word for it.