Per AA's 10-K, in 2024 87% of American Airlines employees were represented by a union[1]. So according to that source it sounds like the people who were fired were union members that didn't pay their dues.
They could surely have paid their dues and left the union and kept their jobs (or could have never joined the union to begin with).
So looks like you're right but there's also some weird language technicality for "closed shop" where it's really a "union shop".
Per the APFA contract[1] employees are forced to join the union within 60 days of assignment as a flight attendant. This is technically considered a union shop (not a closed shop) because it doesn't require people to be union members before being hired.
Under the Taft-Hartley Act a lot of states (and in some situations, court decisions) have made this illegal[2] via right-to-work laws but airlines are covered under the Railway Labor Act (45 U.S.C. § 152)[3] which allows it (upheld by the US Supreme Court in Railway Employes' Department v. Hanson, 351 U.S. 225)[4] .
So, I was wrong and the employees had no choice but continue to be union members and pay dues or be fired because of airline-specific labor law.
They could surely have paid their dues and left the union and kept their jobs (or could have never joined the union to begin with).
[1] https://americanairlines.gcs-web.com/node/42651/html#:~:text...