Yes, LinkedIn currently limits ads to 600 characters. But there's a way to use long-form content as an ad (or "sponsored content").

Instead of starting from Campaign Manager, post the content on your business page. Once the post is live, you can run it as an ad! There should be a button at the top right that says "Sponsor Now" and that allows you to set up your goals, budget, audience, etc!

Want proof? Checkout this LinkedIn post.