The people who supposedly made the app are very close to the DNC (https://shadowinc.io/about). I can't even say I'm 100% buying into this theory regarding sabotage; they probably just made an app that wasn't up to the task at hand.
Every indication I've heard so far (I live in Iowa, not that it matters since this is a national story) is that it was just gross incompetence on the part of the app maker.
They repeated all morning on the radio "No interference or hacking."
Really it's just a classic case of people under-preparing and shit hitting the fan because everything didn't go 100%.
The DNC has a significant amount of influence over the state level party organizations.
There's also plenty of tinfoil hat fodder (some of it very valid) related to how connected the company that developed the app is to certain parts of the national party.
All that page claims is that one or more members of their team worked at the DNC at some point. That's not exactly persuasive evidence that the company is some kind of purpose-built DNC kamikaze cruise missile. It could just as easily mean they've got some people who worked low-level jobs at the DNC, and are now trying to leverage that connection to sound like bigger deals then they actually are.
That's not what the tweet says, the DNC did not suggest they use the app. What the DNC said is that the previous method they were going to use (online voting) could be susceptible to hacking and so wasn't acceptable. The Iowa Democratic Party then decided on another option.