Hm, I would use „hä?“ rather to signal that I don‘t understand the content. I may well have understood the words or sentences, though.

On the other hand „huh?“ is meant in the paper rather for signaling that I didn’t understood the form, AFAIU. For that I would rather use „Was?“ or „Hm?“ in a slightly higher voice, like the German instance at point 25. in Figure 1 suggest.

As a sidenote, I wonder if the examples in the paper are acceptable in semi-formal conversations, say when talking to colleagues at work. In German I would rather stick with „Wie bitte?“ to avoid judgements like in the answers under https://german.stackexchange.com/questions/4876/verbreitung-...