When the speaker underlines the shocking nature of Alcibiades' imprisonment of Agatharchus by pointing out that such behaviour is prohibited by inter-state treaties ([GREEK TEXT NOT REPRODUCIBLE IN ASCII] [[sections] 18]) the detail is not only irrelevant but argumentatively bathetic: if a man imprisons others at his own whim, it is already clear that he is acting in a high-handed and unconstitutional way.
That anecdotes about Alcibiades were already current by the mid fourth century is clear from the Against Meidias of Demosthenes ([subsections] 143ff.), where two of the anecdotes of [And.] 4, the Taureas and the Agatharchus episodes, appear (in slightly different form).
He imprisoned Agatharchus the painter; they say [[GREEK TEXT NOT REPRODUCIBLE IN ASCII]]] he did this too.
Burn's second argument was the different way the two texts report the anecdote of Alcibiades' imprisonment of Agatharchus: whereas in [And.] 4.17, Agatharchus is only able to escape after three months, upon which he is sued by Alcibiades for non-completion of the job, in Alc.
16.5-6 (the Taureas, Agatharchus, and Melian captive stories) appear in the same order as they do in the speech.