Even though you are right, it could be misleading and I don't think it should appear in duolingo.
In a very simplified form (rather for children to learn), when every season has 3 months, March is rather considered a month of spring. Like here: https://3.bp.blogspot.com/-sCI0k9HtqB0/Vd32pVaHbVI/AAAAAAAAAbA/PWvTK4tGUFI/s1600/2.jpg
I'm not exactly defending this sentence, but I understand why someone wrote it this way.
Then I'll defend the sentence. I have heard weathermen talk about meteorological winter vs astronomical winter, so it isn't just for kids.
"Meteorological winter is a three month period that runs from Dec 1st to the end of February. It is the coldest three month period of the year in the northern hemisphere. Astronomical winter is what we all refer to when we talk about the winter season and this is based on when the sun reaches the most southern point on the globe, the Tropic of Capricorn. If you are located right on the Tropic of Capricorn at 12:00 noon on the first day of astronomical winter, the sun will be directly overhead. Also, on the first day of astronomical winter, the sun is at its lowest point in our sky at 12:00 noon and, of course, it is the shortest day of the year. Obviously, it is the first day of the summer season in the southern hemisphere." (Weatherdudes)
"Astronomical winter starts on December 21st, but for meteorologists, it started on December 1st." http://weather.msstate.edu/?p=649
"Different cultures define different dates as the start of winter, and some use a definition based on weather. When it is winter in the Northern Hemisphere, it is summer in the Southern Hemisphere, and vice versa. In many regions, winter is associated with snow and freezing temperatures." (Wikipedia)
There are many more articles available, but you get the idea.
Hmm, interesting. I know, of course, about the astronomical winter, but I'd call the other one 'calendar winter' and for that reason wouldn't treat it that seriously because it even sounds like a very simplified version. Good to know that there's a more scientific way of calling this.
Yes, I've always called it calendar winter, too, but decided to use the more scientific name here.
As one article suggested, how people think about it often depends on the climate where they live. When snow comes in November or early December, most folks would say that winter has come. Many who live in temperate climates like to think of March as spring. The days are noticeably longer, the chance of snow is usually past, and spring flowers are emerging.
I put "March is not a winter month" and it was incorrect. Is this not a possible translation?