You're probably right.
If they don't believe Moses and the prophets (who worked countless physical miracles), neither will they believe if one comes back from the dead.
(I love that passage -- it has meaning on so many levels).
The parable was Abraham talking to Dives, who just went to hell. So the "one giving testimony from beyond the grave" literally referred to Dives, hoping to appear to his brethren to try to convert them.
But it also applies to Our Lord, who worked miracles and rose Himself from the dead. And those who rejected Moses will also reject Our Lord -- once you've rejected the evidence of your own eyes, God's messengers, etc. what's one more rejection?
It's tempting for me (and others) to believe this evidence is especially strong, since it's new and clear -- but you're right, this isn't the first clear evidence we've seen that something is wrong. People have brushed off many other items equivalent to this.