This is true. Zeke Miller of the National Catholic Reporter includes:
"“It’s a great honor,” Biden said after writing in the book, which was opened on a small desk with a large portrait of Benedict in the background.
Biden wrote: “Together with Catholics across the United States, I join in mourning the passing of Pope Emeritus Benedict. He was a brilliant scholar and truly Holy Man. I will always cherish our time together at the Vatican when he was Pope discussing Catholic theology. He was a great theologian and I learned much in a few hours. May his soul rest eternally with the Lord.”"
The National Catholic Reporter is an independent news source, focusing especially on topics such as human dignity and social justice.
The Washington Times corroborates this statement, though not including as much information. The article includes, "Mr. Biden, a Catholic, told Vatican officials it was a “great honor” before writing a personal note in a condolence book for the pontiff.
“Together with Roman Catholics across the United States, I join in mourning the passing of Pope Emeritus Benedict,” the president wrote, according to the White House. “He was a brilliant scholar and truly Holy Man.”"
https://www.ncronline.org/vatican/vatican-news/biden-visits-vatican-embassy-pay-respects-benedict-xvi
https://www.washingtontimes.com/news/2023/jan/5/joe-biden-visits-vatican-embassy-honor-benedict-xv/
