Translation:At least, that is what I have always believed.
Duolingo does know what compound tenses are, for sure. The issue is that we should not see the suggestions given as the exact solution to the sentences we are given for translation.
When you hover on words, what you get is a glossary of various possible translations depending on context and construction.
Homographs exist in both languages, but generally with a full sentence, we should distinguish a past participle from an adjective, in particular with auxiliary "have".
Because it translates all of the following formulas:
he was giving me headaches = il me donnait des maux de tête
he used to walk in the morning = il (avait l'habitude de se promener) se promenait le matin
he would drink wine at breakfast = il (avait l'habitude de boire) buvait du vin au petit déjeuner.
But in this case the "I have believed" structure implies the imperfect -- that I believed it for some time and continue to believe it. That's part of what is confusing -- the English present perfect transliterates to the passé composé (with avoir), but often translates to the imparfait.