Southwest has since inspected its entire 737 Max fleet and Boeing has reminded airlines about "high-wind" inspections.
www.flightglobal.com
There's enough details here to see why Boeing would claim the issue was unique to the aircraft, and why NTSB would differ with them.
TLDR: The aircraft had been flying around with damaged tail controls, probably since it was exposed to high winds (gusts to 73mph) on the ground at New Orleans on 16-17 May. While the problem only became a major one with the dutch roll incident on the 25th, the captain of that flight had noticed a log entry for yaw damper issues, resolved by "resetting a few stall management yaw damper computer codes” - whatever that means, I genuinely can't tell. The NTSB notes that anomalies started showing up on the first flight after Southwestern did some scheduled maintenance checks on the standby tail PCU on the 23rd.
So yes, there were possibly aircraft unique issues, as Boeing claimed. But those issues may have been masked by the tail controls until something related to the checks on the 23rd, in which case there's definitely whole fleet issues to be investigated to understand how the controls were masking the damage, and how the maintenance check changed that. And also potential whole fleet issues related to the gust-damping system intended to stop the rudder being slammed against its stops. (Plus did someone in Southwestern maintenance simply reset a breaker rather than properly investigating the yaw damper anomalies).
So overall, yes, NTSB were right to be pissed-off with Boeing over and above violating investigation protocols. And yes, Boeing's Chief Engineer should have known there was more involved and not tried to dupe the press.
