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It’s true that German doesn’t add an ending to an adjective to use it as an adverb, and also true that most adjectives can be used adverbially. But if you can put a gender, number, or case ending on it, it’s not an adverb/not being used adverbially, and there are all sorts of adverbs without adjectival equivalents. Also, as in English, lots of deverbal adverbs.
More importantly, German primary schoolers are introduced to the concepts of Adverbien and Adjektive as distinctive. So while it’s true that there’s a lot of overlap among adjectives and adverbs, they’re conceptually (and sometimes morphologically) distinguished.