Like in countless other cases Emily Anderson, with her persuasive mistranslation, was able to completely convince her male successors. Her nonsensical "There were shouts of 'Da capo'" duly lead to Spaethling's "There they were: the shouts of Da capo" and Sadie's "When there were shouts of Da capo".
There were no shouts of "Da capo". Shouts of "Da capo" would have been totally out of place anyway at this moment, because this already was the Da capo. Mozart does not refer to what the audience shouted, he is describing it bursting into applause for a second time. Here is the correct translation of the last sentence of this passage in Mozart's letter:
I brought it once again at the end of the movement - and there they went again.
The fact that there is still no reliable and scholarly annotated translation of the Mozart family letters is still a major impediment for Mozart scholarship.