Hemingway was quoting John Donne from 1624 when grammatical practice was different. Spencer seems to be suggesting Ernest should have corrected John’s grammar (and appalling spelling if he’d recognised the allusion).
Spencer Jakab is not saying Hemingway was wrong, quite the opposite. He says:
As for when “whom” is appropriate: It is the correct choice if the word is the object of a preposition or a verb, such as in Hemingway’s “For Whom the Bell Tolls.” The choice should be “who” if the word serves as the subject of a sentence or clause.
It is the object of the preposition for, not the subject of the clause (which is an implied you), therefore it should be whom.