This reference to Emperor Joseph II is one of the best known Mozart-related entries in the diaries of Count Karl von Zinzendorf. The emperor is talking about the Mozart-Clementi duel, nearly a year after it took place on 24 Dec 1781 (see our entry for that date). The sentence in blue above is given in Dokumente (184) and in corrected form in Addenda (40). It has generally been interpreted as if the emperor had something of a fixation on the event. Simon Keefe writes, for example:

The contest clearly made an impression on Emperor Joseph; he was still
talking about it (to Zinzendorf) almost a year later. [Keefe 2017, 45]

However, the surrounding context of Zinzendorf’s entry provides a rather different view.

Zinzendorf had gone to Countess von Pergen’s in search of his cousin, Baroness Louise Diede zum Fürstenstein, with whom he had fallen in love since her arrival in Vienna on 21 Nov 1782 (see our entry for 6 Dec 1782). Countess Philippine Gabriele Johanna Sophie von Pergen, née Baroness von Groschlag zu Dieburg, was the wife of Count Johann Anton Graf von Pergen (1725–1814), who held a variety of important government posts under Joseph II, but is best remembered today as the architect of Austria’s police state under Franz II. The Pergens had three daughters, one of whom died in 1772. The surviving daughters, the ones Zinzendorf heard play the keyboard on 5 Dec 1782, were Marie Therese Josepha, who would have been around 19, and Maria Anna, who was one month shy of eight (see Wurzbach 1870). Louise von Diede was by all accounts a highly accomplished keyboard player. The next day, Zinzendorf reports hearing her play “very difficult” pieces by Mozart and Bach. In Rome in 1788, Goethe heard her play a concerto at the residence of a Roman senator (see our entry for 6 Dec 1782).

In the two weeks since her arrival, Louise had made quite an impression among the Viennese high aristocracy, but the emperor had not yet met her. He seems to have gone to Countess Pergen’s specifically to seek her out, and although Zinzendorf is not entirely clear on the point, it’s likely that Joseph heard her play. Zinzendorf does not mention specifically what she played on 5 Dec, but we know from his entry the following day that she had Mozart in her repertoire.

Thus it was meeting Louise and (probably) hearing her play that was Joseph’s motivation for speaking “endlessly about music, about the duel between Mozart and Clementi.” He was talking to an educated, charming, beautiful, and highly musical woman that he had just met, about a notable private musical event that he had hosted in Vienna, one that she would almost certainly not have known about.