Its because they didn't updated their app for iOS 13 yet. And its not guaranteed even with the update they'll add a dark keyboard because Microsoft Edge updated for iOS 13 but they don't add the dark keyboard and that's the stupid thing about the iOS 13 dark mode. Its up to devs to add the dark keyboard or not, its not system wide.
I suggest you icab mobile for a real dark mode like firefox and they support the dark keyboard