Many bacteria produce antibiotics to outcompete other bacteria, though, so this isn't new or alarming information. Especially given that, as the author admits, the mutation that causes it to produce that particular antibiotic is naturally occurring in S. mutans!

It'd be a more interesting claim if it was argued that mutacin-1140 was particularly dangerous in the relevant doses (and this somehow never came up in the wild-type S. mutans that produce it).