It's under development close to 4 years. If the person has time (or doing Ph.D. or something on the subject), this is a very viable time frame to do all of this.

There are other people's work in it, but it's a one man show mainly.

This is a passion project and it shows.

Edit: Plus, this is the second iteration of the hardware. First iteration is at https://github.com/jankae/VNA

It's not that hard (not that it's easy either!). The individual parts are conceptually relatively simple, the devil is all in the details. For a generalist this is a doable, but likely very time consuming project. I've done something similar (fairly different focus in purpose and specs, but the overall shape and scope is not much off) professionally, mostly on my own.

"Surely that can't be all from one person?"

It's possible. Especially if they aren't a piece of shit like me.

Are we the same person?

I guess that's the existential question - is one pos really any different from anotherin a meaningful way; is any person in general?