Well, you're thinking intelligently by questioning this answer. It's still proper English, but less common. "The journalist explained it well" would in fact be more common and more clear if you were talking about the journalist's explanation of something specific. If the journalist was explaining many things in general, you might hear "The journalist explained well," but even then someone is likely to say, "The journalist explained them well."
I don't think "The journalist explained them well" sounds like a good English sentence. Normally you'd avoid "The journalist explained well" and maybe say "The journalist explained it all very well" or "explained everything quite well" or "The journalist explains things well." I think that's how the verb "explain" works.
This actually happens in Portuguese often, I would say more often than in English. In conversation, when what you are talking about is understood, you do not have to keep referring to the object, like in English you do with "it." -Foste falar com a jornalista sobre essa tua confusão. -Sim e ela explicou bem.
Because "good" is an adjective, which cannot modify the verb "explained." You need the adverb "well": "The journalist explained well." It is the same thing in Portuguese; you cannot say «O jornalista explicou bom.». The correct way to say it is «O jornalista explicou bem.».
That is very true but I personally don't think Duolingo should be taking away points for the student's poor grammar in his/her native language. Even more in a case like this that it is such a common mistake that many people think it is correct. Of course this may be asking too much of the computer that corrects us.
"Reporter" is still not being accepted as a translation for "jornalista".