My understanding of the phrase 'do it well' isn't as an injunction but as a way of saying that if you restrict your program to one thing, you'll have the ability to do it very well. It's to contrast it to a program that does multiple things, not so well.

What I understand flexible to mean is "serving 99% of use case scenarios". Arguably Photoshop does this, Instagram does not. Or maybe a better analogy is Photoshop vs Photoshop Elements, which probably only servers 80% of use case scenarios. Or Chrome OS (80% of desktop use case scenarios) vs Windows (99.9%).

As I said, in this context, it means flexibility, but that flexibility must be tempered with composibility.

Think about how programs become bloated. What helps the core Unix command line tools avoid this fate?