These are different things.

He was not subject to the same constraints and incentives that professional airline pilots are subject to.

For example, without a union and the seniority system, airline pilots that are pressured to fly with badly maintained planes are faced with a major dilemma. They can agree and risk their passengers, or refuse and face repercussions. Their livelihood and social status depends on them keeping their jobs, and the vast majority don't have the resources to withstand lawsuits.

Philip Greenspun can just go back to fly his collection of private planes and helicopters. He faces no repercussions if he refuses or goes to the press.

He has no skin in this game.

And yet it doesn't mean that he can't write a factual article about airline unions using his insider's perspective.

Everyone can write an article about airline unions.

He just doesn't have the insider's perspective.

He can disagree with Sully's reasons why the seniority system is important for safety, but he should at least address it.

This article has a lot of arguments specifically about safety. And even then, unions can increase safety in some domain, while simultaneously decrease safety in other domain and bankrupt host airline.

Moreover, Sully, as senior pilot, was direct benefactor of the union system that exploits younger pilots, so his opinions should be scrutinized first of all.

A better analogy would be the company owner's son who "started at the bottom" explaining why a union is unnecessary.