We learned that after Mark Felt was comatose and could neither confirm or deny.
We never learned it from Mark Felt himself.
We always, all we have is the word of Bob Woodwrd,
https://www.vanityfair.com/news/politics/2005/07/deepthroat200507
Mystery surrounded the identity of Deep Throat - named after a popular pornographic movie of the time - for decades, until Mark Felt admitted being the source in 2005.
“I’m the guy they used to call Deep Throat,” he told the US magazine Vanity Fair in an article that revealed his secret.
Until then, the Washington Post had refused to confirm his identity.
Vanity Fair magazine revealed that Felt was Deep Throat on May 31, 2005
Felt died at home, in his sleep, on December 18, 2008.[96] He was 95 years old and his death was attributed to heart failure.[97]