The first clue is this: "In conversations between The Atlantic and 10 current and former employees at OpenAI..."
When you're reporting something like this, especially when using anonymous sources (not anonymous to you, but sources that have good reasons not to want their names published), you can't just trust what someone tells you - they may have their own motives for presenting things in a certain way, or they may just be straight up lying.
So... you confirm what they are saying with other sources. That's why "10 current and former employees" is mentioned explicitly in the article.
Being published in the Atlantic helps too, because that's a publication with strong editorial integrity and a great track record.