We can understand them! Especially if you are ready and used being hit with interjected English words. It is hard to get used to it the first few minutes (i speak two other languages, language switching is always hard for me), but once you know what to look out for we could be talking interchangeably.

My teacher in high school went there over 40 years ago, and he said, he never had any trouble. Listening on youtube to some samples i still can, and it sounds just like older people from my village, which younger people often can't understand, especially as these older people tend to mumble and speak faster a lot. Pennsylvanian Dutch sound just like this.

Keep in mind though: The region Amish people came from, the Palatinate, was historically a highly fractured region with lots of mini-kingdoms and small administrative clusters. While mostly Protestant, there are villages solely catholic, which were often trying to not mingle with the next villagers. (even it there was a mix of Catholics, Protestant and Jews)

This is mirrored in highly fractured words that are often different from village to village, even if they are just 5 kilometers away. (e.g. the word for soap or apple is pronounced totally different in the next village). This lead to some secludedness and distance which is mirrored in the article and why i think Amish were trying to maintain their distance from the local english population (if you discount the religious component of that population)

Funny sidenote: The highest rate of foreign migrants into the palatinate region --- where the amish came from --- is now from US citizens [https://upload.wikimedia.org/wikipedia/commons/a/a6/Karte_Kr...]