From what I've seen that's kind of overselling it, his own family hasn't claimed that he said that, an anonymous person who claimed to be a friend of his did.
But I may have missed any further developments to actually verify that his family had been told something similar.
Yeah, his family said "He was suffering from PTSD and anxiety attacks as a result of being subjected to the hostile work environment at Boeing which we believe led to his death". As much as a conspiracy for fear of being next could come into it I'd have to believe it'd be more than a lone and anonymous friend of the family that would speak out if confided with this info.
Between that and the 30 some whistleblowers in the last 5 years that two died close to one another isn't all that damning.
And in Barnett's case, he'd been talking since 2019. Even if one thinks Boeing would be willing to have people murdered, the time to do so would be before they became public whistleblowers, not a half-decade later.
But I may have missed any further developments to actually verify that his family had been told something similar.