Buckingham Palace appears to have issued a statement to the press corps - because the information is included in most of the articles covering details of the agreement which were left out of the official statement - that the couple will be known as "Harry, Duke of Sussex" and "Meghan, Duchess of Sussex" once the agreement goes into effect in spring 2020. (One correspondent cited it as "Harry,
The Duke of Sussex", but he seems to be the exception to the rule.)
https://www.bbc.co.uk/news/uk-51107687
It is indeed a departure from the customary usages for British dukes, but it may be a more practical decision taking into account the couple's plans to work in North America. I suppose many Americans and Canadians would find it easier to recognize "Harry" and "Meghan" than "His/Her Grace".
A second possibility, if the expectation is that Harry will also stop using his Prince title (which is yet to be officially confirmed), is that the guidance on calling him "Harry, Duke of Sussex" may be intended to guide the media away from calling him "Prince Harry".
As Mbruno noted, it can indeed, but if the monarch takes the initiative then Parliament's assent is not required. British monarchs have customarily conferred or removed HRH/HH and Prince(ss) titles without referring the matter to Parliament, although on some occasions the government has been consulted.
https://www.heraldica.org/topics/britain/prince_highness.htm