Gli amici tedeschi hanno iniziato a ridacchiare e a parlare ad alta voce in dialetto dopo aver visto una foto del mio barattolo di muesli

My guess would be Seitenbacher Müsli commercial slogan.

SEITENBACHER

It’s very likely a reference to the Seitenbacher muesli commercials. These ads are extremely well-known in Germany and famous (or infamous) for their very strong Swabian dialect and distinctive voice.

So if your friend made a joke in dialect after seeing your muesli, thex were probably quoting or imitating those ads. Even people who don’t speak Swabian usually recognize the reference immediately, because the commercials have been running for decades and are often parodied.