"What 'mature, existing codebase' did Oceanic have that they were able to leverage this project? …What other products did they already have that offered a user interface remotely like their app on iOS does?"
"Existing” ≠ "matching." Oceanic started selling DCs in 1989. That’s over 30 years of Oceanic DC code. As a software guy, surely you understand the difference between backend logic & UI?
Or maybe not.