Your boss told you exactly what you need to do: take more ownership :)

You already mentioned you own one part of the codebase, why not expand on that and find another (hopefully critical) part of the codebase and become an expert/owner on that piece too?

Also, it's not all about writing code. Your boss wants to see you put the needs of the team above your own, so look for opportunities in which you can increase team productivity, reduce risk, and help grow your team in terms of headcount and knowledge.

Also, try not to compare yourself to others, although I know it's easier said than done. Figure out what your goals are and do everything you need to in order to achieve them. Who cares how quickly others progress? Focus on yourself and your own success.