If I understand their outlook, first job is getting people who consistently vote for Dems to be reminded and motivated to go to the polls. 2nd is convincing consistent voters to vote for you (that includes Republicans and third party), a distant last is convincing non-voters or occasional voters. I think the problem with trying to get 3rd party voters to vote for Dems is that the type of person that votes 3rd party is very difficult to convince that you're an ally.

They could completely realign the party platform to fit with 3rd party and non voters biggest issues and most won't shift their vote for many reasons. Disgust for the 2 party system, distrust that the party will follow a more left wing agenda, conspiracy theories, the needs to be contrarian or protect their sense of moral purity, etc.

While I'm not sure I agree with the parties approach to disaffected voters. I do think the amount of investment needed to get those voters is possibly outweighed by the amount of voters you may lose in the process. And that sense of inherent risk is stopping the party from taking a chance. Maybe we get lucky and they no longer see an alternative, but I doubt it.