I worked at a unionized job as a teenager at an industrial plant and I asked how to get hired for real instead of temping and he explained how unions worked there and other plants in town. He said it's extremely neoptistic, they'd usually hire a kid of one of the workers or a friend and the odds of an outsider getting in was thin. They dealt out jobs like a mafia family.
I have a few friends in the trades, all part of a union, and none of them would echo this sentiment.
It's also funny because this is the exact same sentiment people complain about with the corporate world, where it's more about who you know than what you know.
Trade unions and Employment unions are very different.
That said, i suspect the parent story was from years ago, most likely late 90's or early 00's when that type of thing was common
Today most unions are very very very hard up to find anyone willing to work everyone that wants a job, and can actually follow instructions, and show up on time (harder than it sounds it seems) gets a job right now...
I worked at a unionized job. It was not that way for us at all. Perhaps a different industry? It was mostly just protection from shenanigans. Once, for instance, the owner of the company tried not to give us a contracted wage increase - a wage increase he agreed to - and he got slammed by the civil judge. We were paid 3 times the wages that he had tried to steal.