I know he entered the US under Trump. But it is the Biden administration that is bending over backwards to defend the FOI application into how he entered and, more importantly, how he stays in the country - since he may well have entered on an unexpired diplomatic visa which may have been perfectly ok at the time, but should now be on a visa for the ordinary, working citizen he now is, and thus subject to the same rules as all other ordinary citizens seeking to enter the US.
There may be good policy reasons for the US Attorney under Biden defending this so tenaciously, but their arguments and conduct seem to have moved beyond a public interest argument and into a more bespoke one to protect this particular individual from scrutiny. Harry has no direct involvement in the case. He is not a party to it. Which is why the US Defense attorney is not bothered about using the "Spare is fiction" argument, since the revelations inside it are the Heritage Foundation's way of claiming that Harry has waived any privacy argument that might be used to counter the FOI request. They have now moved on to "stigmatising" information being the reason not to comply with the FOI request.
Harry's quasi political activities with The Aspen Institute and the online mental health stuff he has been doing in conjunction with the Surgeon General flies in the face of a person on a UK diplomatic visa. Working, earning money with Penguin, Netflix, Better Up, Travalyst etc would also be illegal, I understand, under such a visa. Conversely, if he's applied for and been granted an ordinary Joe visa, his admitted drug use and potential criminal activity would have had to have been waived by the department. Which begs the question why he should get such a waiver and not other ordinary citizens who have been refused entry? Whilst he may have entered to the US in June 2020 during the last months of the Trump Administration, his working history and other activities in the US is very much under the Biden Administration.