How exactly does "los" equal "on." It doesn't in the dictionary.
It is not at all abnormal to say "Sundays I get up at noon" in English, and I'm a highly competent native speaker. It is perhaps the more common way of saying it. OR did you mean it would sound abnormal in Spanish. I can easily believe that it would sound abnormal in Spanish without the "los" and that one simply cannot say "Sundays I get up at noon." Often other languages -- for example French -- require the use of an article when English doesn't.
It is one thing to say that if you were to translate "Sundays I get up at noon" INTO Spanish you have to say "Los domingos." (Could just say in Spanish "Domingos" without "los"?)
It is a quite another to say that "Sundays I get up at noon" is not an acceptable English translation when there is no difference in the meaning in English.
I play with trying alternative phrasing in english sometimes, to see what Duolingo will tolerate and "I get up at twelve o'clock on Sundays" was not accepted. I have had no problem with writing "twelve" instead of "noon" before. So I think the issue this time was with the "o'clock" which we use quite a lot in British english.