Interesting! I'm willing to give Courvoisier a bit of doubt: there have been too many exonerations of "confessed" murderers in the US, who it turned out were demonstrably innocent. I can only imagine that the coercion of the 1840s would have been more compelling.
Without giving too much away, there was no coercion here (at least as related by the author); he was, in fact, on track for an acquittal, when last minute evidence was turned in by a citizen that identified him. He'd privately confessed to his attorney at that point, but had no intention of changing his plea. It's not out of the realm of possibility, of course, but I can't see an upside to him taking this path.