"The letter was crafted specifically with the potential of public consumption in mind because the claimant appreciated Mr Markle might disclose it to the media," the lawyer told the Court of Appeal.
The publisher was ordered in March to print a front-page statement stating it had infringed the duchess's copyright, but the statement has not been published pending the outcome of the appeal.
Three senior judges preside over the appeal, which is expected to last three days. A ruling is due at a later date.
Meghan, a former star of the American TV legal drama Suits, married Harry, a grandson of the Queen, at Windsor Castle in May 2018.
Meghan and Harry announced in early 2020 that they were quitting royal duties and moving to North America, citing what they said were the unbearable intrusions and racist attitudes of the British media.
'If you need be, I'll be there'
The court heard arguments about the People magazine article which first made the existence of the letter public, the Telegraph UK reports.
Caldecott argued Thomas Markle had to defend himself "only by disclosing the contents of the letter".
Citing pre-wedding texts to the Duchess of Sussex, he told the court Thomas told her:
"It's all so close now and I can't wait to walk you down the aisle," later adding he had undergone heart surgery and was unable to fly.
"The doctor will not allow me to fly so of course I can't come. Love you and wish you the best of everything."
The lawyer said he later asked her via text: "Who will be giving you away? If you really need me, I will come. I'm sorry about all of this."