This wasn't a big crossover event, there was no Flash and Arrow together action, but it still served as a shared-universe episode nonetheless thanks to the arrival of Diggle and Lyla. In truth, King Shark could've worked without them, but by bringing them into the fold it gave the episode a bit more weight, and served as a nice reminder that these two shows are set in the same world, with characters who know each other and help each other out. It meant we get some extra Lyla action, and a nice resolution to the death of Amanda Waller, something that Arrow had failed to follow-up on. Moments like this serve both shows well, and hopefully it's the kind of thing that will continue to happen. Also: Diggle being stunned by Barry's speed will never not be a brilliant, hilarious thing.