"Deniers assert that the Holocaust is a fabrication or exaggeration,^[3][4][5]^making one or more of the following false claims:"

and then an incomplete list of holocaust denialist claims. Thats from your own link

J K Rowling is claiming the holocaust is exaggereted in them tweets, therefore: denying part of the holocaust as defined by the above.

and holocaust denial is an academic term with a pretty precise definition, as much as it is a colloqial one.