Be wary of putting too much faith in pre-election polls. My understanding is that people tend to fall back to party loyalty when it comes time to actually vote. You probably also have a reverse-Obama effect. There were people who said they'd vote for a black man for president who in reality wouldn't. By the same token, I expect there are people who say they don't support Trump but secretly do.
I am not sure how that graph illustrates where the undecideds are going. It simply says that 84% of people are voting either Dem or Rep. How does that translate into: "Clinton's lead is becoming safer"? In a buried sentence: "With more voters committed to one of the two major-party nominees, Trump simply has fewer people he can appeal to in order to make up his current deficit, which makes Clinton’s lead more secure."
What it says is that the undecided voters are falling in the Clinton camp, and that Clinton has a strong lead in the polls. If all the votes fell 50/50 Democrat/Republican at this point, Clinton would still have about a 5% majority. If all the votes fell third party at this point, Clinton would still have about a 5% majority. About fifteen percent of the electorate is undecided at this point. Clinton has about a 5 point lead. Trump has to pull in undecided voters by a two to one margin in order to make the election close. That would require an impressive turnabout of public sentiment at this point. Also consider: this clip has existed since 2005. It's been in somebody's Avid for ten years. And there's a debate day after tomorrow. Who the hell runs this shit on a Friday afternoon... ...unless you're trying to make the whole Alicia Machado gambit look like a warmup act? The debate is going to be interesting.
I was curious why this was released on a Friday.