This doesn’t mean that Demosthenes said such a thing (which would certainly have been in Greek anyway), but that he had such a thing.William Martin Leake dixit:the Athenians, on the death of Alexander, were excited by the “sibi mortifera facundia” of Demosthenes to renew their struggle with Macedonia.
All online references to that phrase seem to be to the following line in a book called Travels in Northern Greece.
This doesn’t mean that Demosthenes said such a thing (which would certainly have been in Greek anyway), but that he had such a thing.
Mortifera facundia is ‘deadly eloquence’. I don’t know why sibi, ‘for himself’, is there. I imagine it is plucked from some longer quotation, with which the reader is expected to be familiar.