Knowledge about instruments and their construction has increased enormously over the last decades. The harpsichords Landowska used were made by piano builders trying to use methods of piano construction, which in the end did not yield satisfying results. (These instruments mockingly were described as "egg slicers" in Germany) It has a lot to do with string tension and tuning pitch, as well as soundboard construction, housings and pinboards. Only in the 1960's did builders start to study and meticulously copy historic harpsichords, and they learned a lot in that process. They now are on the same level as the great instrument makers in the 17th and 18th centuries. And they now know the differences between different historic workshops and regional styles etc. The sound of Landowska's or Ruzickova's harpschords has a lot to do with this. (I just got a 10 CD box with remastered Landowska recordings but still have to listen to it.)
Plus, most engineers and record producers stll have to learn how to record these instruments properly. They love the sound ambience of churches, but harpsichords are not suppoosed to sound good in there. Saloons, private apartments, much smaller rooms with little reverb, and intimate acoustics are appropriate. Many of Gustav Leonhardt's best recordings were made at his house in Amsterdam.
Overtones are very important, for the timbre. Church reverb absorbs these.
As I said, a very complex subject.