That would be stupid of them. Maybe they didn’t find anything useful on the phone. Or maybe they have already made arrests.
Of course they didn't find anything useful on the phone. There never was anything useful on the phone. Anything useful, if it was on any phone at all, was on the personal phones which the shooters destroyed prior to their crime rampage, not on this government-issued work phone.
Personally, I think the FBI is just saying that they cracked this phone as a face-saving excuse and a final FU to Apple for humiliating them. They can safely lie about it because they're never going to have to produce any evidence (which, as explained above, doesn't exist in the first place) from this phone.