Interesting. Annehmen has two meanings: to assume/suppose, and to accept. However, the latter only applies to accepting certain things, like a wedding invitation, and primarily tangible items, like a gift or money, but NOT something intangible like a concept, an idea or a reality. Seems like DL has only considered the first option, and it may well accept "I suppose so" as an alternative translation.
Worth reporting this as an omission.
According to pons.de, "I suppose that" should be accepted. https://de.pons.com/%C3%BCbersetzung?q=Annehmen&l=deen&in=&lf=de