Brahms: Ein deutsches Requiem, Op. 45: I. Selig sind, die da Leid tragen

Last week, I listened to Brahms' Ein deutsches Requiem, Op. 45.  I've listened to it only a handful of times (eight, according to my records), so I'm not that familiar with it, but I did notice a small feature in the first movement (Selig sind, die da Leid tragen).

Here are the first few bars of the string parts:

[source]

The third bar of the cello part and the fifth and seventh bars of the viola (Bratschen) part match the first bar of a tune that Haydn wrote.  Here's the first phrase:

In The Lutheran Service Book, this tune has the title "Austria."

According to Britannica, Haydn's tune was paired with the text of the "Deutschlandlied" in the 1840s, although it didn't become the official national anthem of Germany until 1922.  Brahms' requiem is from the 1860s, a few decades later.  It seems, then, that he took the opening notes of the "Deutschlandlied" and put them here at the beginning of his requiem, as if to indicate its "German-ness."