It's a
famous story about a monkey's paw that grants wishes, but only in a horrible or undesirable kind of way. For example, in the story a man wishes to have £200, and the next day his son is killed in an accident and the company he was working for pays £200 to the survivors, i.e. the father.
Likewise here. Nationalism is a disease, Einstein says, with the implication that we should combat and defeat it...and Gumilyov does so by introducing
super-nationalism, which is not at all what Einstein would have wanted.