It definitely is NOT one of the hardest in solo piano repertoire, but it is quite difficult. Liszt wrote many other pieces that are much harder. Better candidates for the title would be Gaspard de la Nuit, Trois Mouvements de Petrouchka, Alkan's Etudes op 39, etc. Difficulty may be subjective, but no knowledgeable musician thinks that HR 2 is the hardest (especially in terms of technique).
So, in short, wiki is not very trustworthy.