Anecdote: in one of the law firms whole departments were sent to LWOP due COVID-19 and department heads had to do all the work themselves. They found out that they are perfectly capable of singlehandedly doing the job of the whole department.
My take: if your boss can do your job, you're not needed.
If your boss can do your work, has the time to do your work (eg there isn't enough work), the work doesn't need to be done in parallel, and can do similar quality work, and is willing to do that work, you're not needed.
That's the takeaway, without oversimplification, which makes sense.
It's actually an interesting point that. Oversimplified a bit, I feel there are two different ways that people measure their worth in the workplace. There is "no one else can do what I do" and then there's "I make sure that everything I do can be done by someone else".
Personally I much prefer the latter.
I can program in ASM, since I did it in college. That's a different assertion from "if a colleague can do your job, you're not needed". ASM programmers are still needed. Also, there's the whole "making a baby in 1 month with 9 wombs doesn't work".
If he has time to do your job. My boss can code, and very well. But he doesn't have time to do so.