How to lead when working with unreliable/incompetent engineers?

The Meaning Of Leadership

A Director told me Leadership means making sure the project is successful no matter what happens.

This is more or less correct. This is what I have been told many times across Meta and Robinhood working on senior and staff scope.

What separates the decent engineers from the incredible engineers is their ability to come up with more graceful solutions than "Fine, I'll do it all myself", which I'll cover more later.

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?

1. Uplift the low-performers as covered before
2. Find a mentor to do #1 for you - As I was growing to E6, I got to this point. I would have my mentees mentor other engineers on the team, allowing me to mentor engineers on how to mentor other engineers. More scope for me!
3. Escalate to managers, which can produce the following results (alongside others):
   1. The feedback finally lands and the low-performers improve
   2. They find other resources (i.e. bring in other engineers) to offset the gap
   3. They reduce scope on the project so the low-performers' contribution isn't as needed

Of course, it's possible that all of the above fail, and you need to do it yourself. But what's important is that you tried the more sophisticated solutions first.

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)?

I recommend this discussion on how to improve a less-than-friendly relationship with a core XFN partner.