The article you link is about acquisition of past tense formation in Danish, where some verbs have a past tense ending in -ede, like bade/badede (bathe/bathed) and some others end in -te, like kalde/kaldte (call/called) and yet others are completely different. Those verb endings are frequently reduced in speech, which makes them sound similar or identical. In the study, children who used more different reduced forms were more likely to use a -ede form for a verb that usually goes with -te.
An adult learning a language is more likely to learn the "proper" form in a controlled environment first and then learn to slur their verb endings later when they reach a level where they can have casual conversations with native speakers. Compare also English "would of" vs. "would've," which native speakers seem to confuse a lot, although they're obviously different to someone who first encountered those words in writing.