This. I'd conjecture that this has led to OP becoming overconfident.
I also think that OP should not be a manager. If one dev is doing 70% of the work then that's either bad hiring practices or micromanagement where other team members are not trusted to deliver complex projects.
Someone with the management sense would’ve already started thinking and implementing that and not continue to code moar.
You don't know they haven't already starting thinking/implementing ways to address those risks. And... it may also be that there's not enough political power to make changes substantive enough to address the systemic risk. OP mentioned 300+ developers on a team - that's a large place. There may simply not be the ability to wield enough influence to make real change to reduce this sort of risk across the board.
Documenting your own stuff, coming up with some better ways to share knowledge, pairing, etc - that can help reduce some localized risk, but you don't know that they've not done this.
But post also points to being lonely of sorts and a sense of dispair. As your last paragraph describes, if OP pair programs, does some mentoring, etc, why would they feel lonely and post on the internet instead of chatting it through with the people at the company?
If so many people are coming to him/her for things, they def have some implicit power. Not knowing how to constructively use that power also lead to my support of the “don’t be a manager” post (yet). If OP figures out how to scale themself out, then by all means…