Actually, that German example was pertinent to the discussion as an example of separating the idea of location from other ideas, much as ser and estar separate ideas of location and origin.
"Ich laufe ins Haus." means I'm running
into the house, whereas "Ich laufe im Haus." means I'm running around
inside of the house. Use of the dative case with positional prepositions invokes the older locative case, which indicated location ("im Haus"), whereas use of the accusative with the same preposition indicates motion in relation to the object, such as transitioning into the house ("ins Haus").
The Proto-Indo-European is supposed to have had no prepositions; didn't need them since the 8 cases served that purpose. The pattern that we see is for some cases to get absorbed into another case along with the increase in use of prepositions. For example, Anglo-Saxon (AKA Old English, extant until after 1066) still had cases, but at that time the instrumental case was being absorbed into the dative. Then after the Norman Conquest, English became very heavily French-ified, pretty much killing case in English among other things *.
It makes life a lot easier to read a language that still uses cases. I remember a few times in French class where the word order was a bit strange and I couldn't begin to figure out who was the subject and who was the object. The "problem" for students is that they have a number of endings to learn, one of the things that Mark Twain lampooned in his essay (grammatical gender being the other, but many languages have that too, so German is not special).
As for German not getting it, of course not, since they are native speakers which makes it all perfectly natural. Native speakers of English are the same way about their language. And when a native speaker is trying to teach his language to a foreigner, he will not understand why the beginning student makes the kind of stupid mistakes that he keeps making. That is why my Russian prof maintained that a first-year language teacher should not be a native speaker.
To illustrate, there's a joke in Robin Williams' "Moscow on the Hudson" that I think most of the audience didn't get. It caught my attention, because years earlier a Yugoslavian friend had said the same thing about her mouth hurting at the end of English class.
I don't think that Mark Twain was making fun of German itself, but rather of English speakers trying to learn German and the funny ideas about language that they carry with them.
Footnote:
About 25% of English vocabulary is still based on Anglo-Saxon, with much of the remaining 75% coming from French. That is why it's easier for an English speaker to learn French vocabulary (and Spanish because of similarities with the French) than German vocabulary. At the same time, the English verb system is still very Germanic, so it's much easier to learn the German verb system than the French, Spanish, or Italian.
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