I agree with this. Precising platonic for a friendship doesn't really make sense to me, unless people would assume it is not platonic. For example :
- I'm really close and affectionate with one of my friends, and people often assume that we're together. In that case, I might say it's a platonic friendship
- since the word friend is part of the words boyfriend and girlfriend, I can understand that people want to be more precise. Especially if the friend in question is neither a boy nor a girl
- it could also be use with irony, since insisting on the "platonic" makes people wonder even more if that's actually true