The Iowa caucus is not a paper ballot. People stand in groups in a large multipurpose room and raise their hands for who they vote for. Depending on the viability threshold, people move to new groups and are recounted.
The exact rules are complicated and the process is run by unpaid humans. Moreover, in 2020 everyone has smartphones to post embarrassing mistakes on social media. And this year is a very crowded primary for the democrats, raising the chance for error.
Under this theory, the app is a convenient scapegoat to hide the fact that the process is inaccurate and bad. For politicians this seems pretty convenient. Who would you rather blame, and decades old tradition or an app contractor?