At the root of “allusion” is the verb “to play” (from the Latin, “ludere”). A calling into play is not the same as referring to or mentioning, and it need not be covert or indirect. Like other poets, Keats sometimes puts his allusion in direct quotation:

Keats follows Twelfth Night. Or, What You Will. With any allusion, the play's the thing (wherein I'll catch the conscience and the consciousness). The concept of allusion and its application ask to be flexible, as with the due amount of play in any steering wheel.

Keats alludes to mythology, history, topical circumstance, and so on; this essay attends to the calling into play of the words or phrases of a previous writer. An allusion predicates a source (no coincidence); but identifying a source is not the same as postulating an allusion, for a source is not necessarily called into play by its beneficiary. What goes to the making of a poem does not necessarily go to its meaning. Sometimes readers will disagree as to whether a line of Keats had its source in, say, Hamlet; sometimes, they may agree that such was a source but disagree as to whether he was alluding; and often readers will disagree as to just what they should make of what Keats made of that which he alluded to. This, not because in criticism anything goes, but because much goes. Poems have a way of being undulating and diverse.