I don't see how what you're saying is at odds with the author. At no point did they say Vonnegut was a failure before Slaughthouse Five. Only that he, like many others, didn't produce their opus until later in life. This isn't just limited to writing. There are examples in all fields if you look, both creative and commercial. This idea is definitely at odds with a lot of current SV rhetoric.
Your point, that many people don't produce their magnun opus until later in life, is definitely at odds with a lot of current SV rhetoric. And it's a good point.
If the article had tried to make your point, it would have been a much better article.
Instead, it made a different, much less true point, and had to contort Vonnegut's biography to make it.
"His career looked like a sequence of failures until it suddenly wasn't" is just not true of Vonnegut, not true of Galilei, not true of any of the other "examples in all fields" cites in the article. All of them are people who consistently produced great work from early on, well before their 40s, and then produced a magnum opus that really stood the test of time.
