How to lead when working with unreliable/incompetent engineers?

I'm an E5 iOS engineer at a Big Tech company. I'm struggling to understand what leadership means. A Director told me Leadership means making sure the project is successful no matter what happens. As a result, I often end up doing other people's jobs for them (e.g., when a backend engineer struggles to get their piece working, I often hand-held them and have even created backend PRs to unblock some projects even though I'm an iOS engineer). However, a mentor told me that E6's don't do the work themselves but delegate it to others to scale themselves. If this is true, then what does an E6 do when working with engineers who are either unwilling or unable to complete their deliverables in the expected timeframe and quality? This becomes even more challenging when working cross-functionally with engineers who are not on my team. Also, how do you lead when you're not the DRI (Directly Responsible Individual) / tech lead for a project (e.g., when the tech lead is an E6 on another team, but is not setting up the team for success by refusing to communicate requirements clearly)?