Try to bear this in mind, polls dont matter much unless they're specifically targeted at individual states
If you have ANY idea how the electoral college works, you would understand that popular vote means dick all. So for instance, Clinton may overwhelming win commiefornia, which has a very large population, maybe she'll win by 70% to 30% there in fact. But it doesnt matter, if she won by 51% she'd still get the exact same electoral votes either way.
Conversely, democrat votes in states like Alabama, Georgia, MS, LN, Montana, etc mean nothing because they will always be overwhelmed by the republican voters there. Those states dont go blue. But they are also lower population states so if its just done as a "random survey across the country" you may end up with more democrats, or more fuckwits from commiefornia just due to how statistics work.
In short, the majority of the country right now, is irrelevant. The southeast, the west coast, much of the midwest, all irrelevant. its why presidential candidates dont campaign there during the general, because they arent going to turn those states. They focus on the rust belt, FL, Virgina, and other "swing" states which have a close enough 50/50 breakdown of D/R that they could go either way.
not to mention, the poll you cited? Yeah it just said "registered voters" and it also mentioned millenials. We know millenials dont fucking vote, they didnt even get out to vote for their gibs candidate sanders. Their margins are irrelevant. Hell look at brexit, same shit there. They all talk a big game, but when it comes time to show up to vote, they dont.
But as far as I can see, theres no information there to suggest they got a fair assessment of voters. They didnt list democrats vs republicans vs indies total in the votes, they didnt list area of the country voters came from. nothing. For fuck all we know it was 50% democrats answering, and even then they didnt get anywhere near a large clinton lead. And again, if they DID get a fair assessment of what the popular vote will be, it doesnt matter, because popular vote doesnt elect the president, electoral vote does
heres a fun fact. If Romney, a hugely unpopular candidate, had gotten just half a million more votes between a mix of florida, ohio, new mexico, and virginia, he would have won the presidency, but STILL lost the popular vote by over 4 million votes.