Probably, the wiki adopted the correct romanization of tsumidango's hiragana username since it's what s/he mostly use.
The reason why s/he use Tumidango when romanized, instead of "Tsumidango", is because when a jp user type つ in their keyboard in romaji input, they use /tu/ instead of /tsu/. This goes the same for "ち", instead of /chi/, it's /ti/. Also, in the hepburn romanization, this is recognized as /tsu/.
You can use Tumidango if you want. It doesn't matter. As long you give him/her credit for the model, I'm fine with it.
However, let me tell you something. There is always a lot of research behind every page I create on this wikia. And I've always used the name in the read me. Read me is signed up as "つみだんご". So, I've used “Tsumidango” because I thought it was the most correct Romanization. Don't think I just came out with this and let's used it cus YOLO. Ok?