If you listen to the audio, which ABC helpfully provides in the story, it's abundantly clear that this is a standard-issue poll -- the sort of thing that political campaigns do all the time. Polls conducted by/for a campaign are much more than just the standard "Will you vote for Candidate X or Candidate Y on election day." They are attempts to understand the electorate at some deeper level -- what do voters want to hear more of in terms of a positive message from your candidate and, yes, what sort of negative attacks would work best against your opponent.
Judging from the call, the negative messages being tested by Clinton against Sanders are totally normal fare for the heat of a presidential campaign. This one -- "Bernie Sanders is making big campaign promises that will cost up to 20 trillion dollars. The New York Times said his plans are not realistic" -- might not sit well with you if you are a Sanders backer (as the guy on the phone was/is), but it's far from out of bounds in terms of modern politics and polling.
It's most certainly not a push poll -- a much-misunderstood term that doesn't deal with actual polling at all. Push polling is a misnomer. What it is meant to describe is more accurately called negative phone calls. Someone calls you and tells you a bunch of negative stuff about a candidate. Then they hang up. It's not a scientific survey. It's just a negative phone call designed to push negative -- and often false -- information out to the voting public. John McCain having a "black" child is the sort of thing moved via push poll.
That isn't what Clinton and her campaign are doing here. Not even close. If a poll like this was a push poll, then every candidate who has ever run for anything is engaged in the dark art of push polling.
We in politics and political journalism do ourselves and the people who read/watch us a disservice when we throw around words like "push poll" for something that clearly isn't one. Let's stop.