I hate to be critical of what's a rather impressive piece of work, but a lot of the reason for that length is just a lot of repetition, e.g. https://github.com/xwmx/nb/blob/master/nb#L10423
Note that Bash is a weird language that doesn't provide language features found in normal languages, and this targets old Bash which doesn't even have associative arrays, so there are things that might look "wrong", but aren't necessarily in this context. That's a big part of what makes it interesting to code in.
I am not bashing Bash ;) but genuinously interested in the motivations as a programmer. You have stated that the Bash shortcomings are "interesting", so I guess it boils down to some kind of "who cares, let's do it with Bash".
I tried nb yesterday and damn if it had a native capture tool for macOS and iOS this would be the perfect Evernote replacement for me.