Ok, then how about this: "How was it that you knew her when you were at school?" "She sold me yoghurt." There I'm talking in the habitual aspect which is imperfective in Polish, so I would say 'sprzedawała' rather than 'sprzedała' but in English I definitely wouldn't say 'was selling'.
I believe that's a less formal /less correct / dialectal / archaic? construction, or at least it's less common.
I've realised recently about the past tense in English that actually despite our 'present have + participle' tense-aspect being called 'perfect' and quite possibly being congatic, if you can still use that term to talk about grammatical constructions, with the French 'present avoir + participle' and similar things in presumably several other languages which they use for the 'perfect' tense-aspect, in English it's a lot more commonly if not always used for actions in the past that still have some kind of effect on the present, e.g. 'I have eaten' (I have eaten recently enough that I'm not hungry at the moment); 'She's always wanted to do this' (she still wants to do it/that's why she's doing it now); as opposed to the simple past tense; 'I ate' (a nice meal yesterday); 'she always wanted to do this' (so I don't know why she doesn't want to now / ..as a child, then she apparently didn't for a while but now she does again); which seems to be used for actions that happened and were finished (whether they were instantaneous, ongoing or habitual), which seems to contrast with the 'perfect is for completed actions, imperfect is for ongoing or incomplete actions' that we get taught at school when learning other languages.
I believe this 'difference' may be mainly or at least partly due to this wording used by foreign language teachers when introducing the idea to pupils - not that it's the fault of the teachers themselves or the syllabus. Or perhaps the wording used by linguists for describing these tenses and aspects in English is not quite right yet, which would be understandable because tense-aspect-mood is complicated in general even without English having lost basically all its verb endings.
I think I'm rambling now, but I suppose really my point is that the 'present have + participle' and simple past aren't always used for perfective and imperfective respectively, and in fact what I understand the difference between these two to be is usually marked by something else in English (e.g. 'present have + past participle' vs. 'have been + present participle', or simple past vs. 'past be + present participle' or 'used + infinitive').
Having said all this, I'm not sure now that it justifies changing any accepted answers on Duolingo because as you've pointed out before Marek, and as other mods have said on this and other courses, if Dl really accepted all possible translations for all sentences then for things like this it would be harder for users/students to learn the right use of the language they're trying to learn with just exercises.
Anyway I was really interested when I realised this so I thought other linguaphiles might be. Have a nice day!