As a preface, I think this particular Harkle Honking is bit of a non-story. I think the $400/hour rate is derived from the minimum $1200 half day fee, which in LA dollars is not much. I've rented deposition spaces in LA for same 3-hour span for a lot more.

Anyway, the Cynical Me says that they got it for free in exchange for "product placement". The Lessor is already HARK'ing about the latest "celebrities" (the HARKles) to use the facilities.

The Alibi Me can think of several "reasonable" alibis, errrr explanations (which are not mutually exclusive of using the above Mulroney Match):

1. The McMansion is being redecorated and is unusable.
2. The deed restrictions in the McMansion neighborhood do not allow for commercial filming (except for purposes of sale).
3. The Harkles really don't own the McMansion, and the lease (or loan out) terms do not allow for commercial filming.
4. The rental venue is easier to equip for video lighting.
5. Some other event had or was about to be filmed, so lighting was already in place (see, no. 4, above).
6. With regard to Nos. 4 & 5, it could have been a unrelated event, another event of the organization, or even a Netflix, Oprah or other inerview of the Sussexes.

If I had to choose, I'd pick Speculation No. 6 with a dash of Mulroney Match.