It makes sense from a privacy perspective. Both of the companies are probably aware of the fact that Apple Maps are really fucking bad at the moment. So was DDG at the beginning. It's incredibly difficult to offer a product that matches Google's when you're two decades behind. The only way to improve is to gather more data. You don't need to collect personal data in order to improve the service, just data in general.
Including Apple Maps in a privacy-first search engine gives Apple the marketing boost in their target market: relatively rich people that do have something to hide. If you use a privacy-aware search engine, and you see that search engine partnering with Apple, it's easier to believe that Apple truly is privacy aware. I still have my doubts, but they're slowly but surely diminishing.
From DDG's perspective, it gives them relevance. They're no longer just a small player in the market trying to make a name for themselves. They're big enough to be able to partner with Apple. This isn't their first collaboration neither: Safari was the first major browser that included their search engine out of the box (Firefox was the second one, about two months later).
It also makes perfect sense for them to stick together, because their ultimate goal is the same: to offer an alternative to surveillance capitalism. It still doesn't make much sense in the short run, but it makes perfect sense in the long run. The more people distrust Google/Facebook/Microsoft/Amazon, the more they're gonna look for the alternatives. DDG, Apple, and similar companies just need to be stubborn. The market will find them, not the other way around.