It was the former: they campaigned exclusively on why Brexit would be bad. And moreover they argued exclusively that it would be bad due to economic impact.
It's not something you ever read here on HN and it's very surprising/unusual, but for people who made their decision based only on the campaigns, hindsight shows that voting Leave was entirely correct and voting Remain was incorrect. You'd never be able to have that level of clarity over something like an election because there's too much vagueness involved, but the specific structure of the Brexit vote and campaigns actually allows an objective evaluation.
The reason is that both campaigns made unusually objective and testable claims. On the Remain side they predicted a huge and immediate recession if Leave won due to uncertainty, they predicted enormous "emergency" tax rises to compensate for that, they predicted the UK would not be able to stay in the Horizon research framework, and more generally they predicted that the UK would underperform relative to the rest of Europe due to new trade barriers hurting trade with Europe. On the Leave side they stated the UK would be able to implement an Australian-style points based immigration system, that it would allow for new trade deals, that it would let the membership fees be reallocated to general healthcare spending, that it would allow divergence from EU law and that it would let voters "take back control". Both sides made no specific predictions about what would happen in the Remain case iirc, because that was assumed to be the status quo.
Well, Leave did win, and that lets us test these predictions. The 500-800,000 job losses due to an uncertainty-created recession never happened, the UK is still in the Horizon research framework (after leaving and rejoining), and the trade ratio of EU:ROW for the UK hasn't changed. In fact there's been no discernable change in EU/UK trade whatsoever, implying the single market had no benefits. The UK's performance has matched the rest of the EU as well. So basically all their main predictions were wrong.
On the Leave side, the UK does now have a points-based immigration system, it does have new trade deals, the membership fees are now being spent on public services instead, the UK is diverging from EU law (albeit slowly), and so on. The main disappointment for Leave voters is that mass immigration ideology proved so strong that the Conservatives chose political suicide and set the points threshold so low that immigration soared instead of reducing, this is technically not a failure by Leave who only campaigned on having a controllable system. The implicit assumption was that politicians would prefer to keep their jobs and set the points threshold high than destroy their own party; this assumption was wrong and now voters are "taking back control" by wiping out the Tories and moving votes to Reform... something that would have previously been impossible (as Reform's manifesto wouldn't have been legal).