The boundary is genuinely disputed and always has been. Some rhetoricians treat synecdoche as a species of metonymy; others hold them separate; a classical strand, Quintilian's, collapses both into metaphor. Handbooks contradict each other, and the contradiction is old rather than sloppy. If two well-read people disagree about whether 'the crown' is synecdoche or metonymy, they are re-enacting a live classical argument, not making a mistake.