That genuinely doesn't seem to solve anything to me.
Sure you can generate all these secret codes but then why wouldn't a briber ask for you to take a picture or video of the screen with all the codes and secret? OCR and computer vision is quite good nowadays and most people are carrying a video camera in their pocket, so the process can potentially be scaled. Bonus points if its install the Purple App and ask the voter to point their camera at the screen with all the codes. Double bonus points if the app generates a nice easy password for the used to plug in to be used as your secret.
And the thing is that it doesn't need to be super accurate. Even if its only budgeted with $10 million worth of $100 gift cards and it's only about 70% of the cards were getting the desired outcome, that's still 70,000 votes going purple. Especially if you limit it to being the first 100,000 confirmed voters, you'll still get people participating if they think there is still hope for getting a card. Even more if you're convincing voters that are only voting for the sake of a gift card and don't actually care about the result of the election.
And ultimately that's just one of several attack vectors I can think of. And I'm not a smart person; I'd go as far to say that I'm actually pretty stupid. I can't imagine what a room full of actually smart folks with NSA-like budget and NSA-like permissions can come up with. Remember the gigantic mess with Dual_EC_DRBG in the FIPS 140-2 standard?