Tautology is an unnecessary repeating of the same idea in different words, as in the sentence He sat alone by himself. In this case, I agree with the explanation that alone is redundant.
However, depending on the case, I think it is not always redundant to repeat the same idea in different words. It occurs when words such as adjectives of the language are insufficient to fully express the situation or emotion. I think this is a phenomenon that occurs not only in English and Japanese but also in any language.
"色々｛いろいろ｝な果物｛くだもの｝ various fruits" or "多彩｛たさい｝な果物｛くだもの｝ variegated/colorful fruits" is sufficient to express the above picture, but how do you tell the picture below to him who is supporting you? You would realize soon that adjectives such as "色々｛いろいろ｝な" or "多彩｛たさい｝な" alone are insufficient to convey your excitement to him.