No, that is not true. The main translation in all sentences with tröja in this lesson is sweater, but jumper is accepted everywhere too. The system tries to match whatever you input to the closest accepted answer, which means you may be shown answers with jumper or something else depending on what you input.
Fun fact: both jumper (pronounced as a Swedish word) and sweater work in Swedish too. Jumper (en jumper, flera jumprar) appeared in the 1920s but was used a lot in the 50s. I have a feeling it's getting less common these days, whereas sweater may be getting more common. The two mean the same in Swedish as far as I'm aware, but sweatshirt (which is also used in Swedish) has a slightly more specific meaning.
However tröja is the most common word.
The reason it was jumper is probably because Sweden is close to England. There is no such word in British English as 'sweater.' At least in Australia you would never hear an Australian say it. But because everything is so Americanised these days it's probably common for Europeans to learn this.
jumper was borrowed into Swedish from English in the 1920s. I've always thought the English word came from it being a piece of clothing that makes it possible for the wearer to move freely and jump around if they'd wish to (many older kinds of clothes would restrict the wearers movements more), but when I looked it up I found I was wrong, they say the origin is unknown but believed to be from French jupe, 'skirt' (which in turn stems from the Arabic jubbah 'long cloth coat').
For those who are struggling to differentiate between skjorta and tröja:
- Tröja is a jersey (like the kind of jerseys worn to school; pull over in some countries).
- Skjorta is buttoned shirt (like the shirts worn to school or formal occasions with a suit).
- T-shirt is the same in English as it is in Swedish