The New Republic is a reliable magazine. It names the doctor in the article. Dr Zeke Emanuel is a doctor who worked for Biden, not as a Biden's personal doctor but as a policy advisor for COVID. He even has a wikipedia article.

https://en.wikipedia.org/wiki/Ezekiel_Emanuel

The title is misleading click-bait. The title says "Doctor who worked with Biden says he had cancer for years". By this title, you are misled into thinking that the doctor knew that Biden had cancer, but the title doesn't say that. The doctor worked for Biden's covid policy panel and Biden has late stage cancer, therefore this doctor made the inference now that Biden must have had cancer for a long time.

The article does not say that this doctor was aware of the cancer previously.