The essence of the trick: make the sign bit the least significant bit. Then all the leading bytes for integers of small magnitude remain zeros (not 0xFF like in two-complement integers), and compression becomes easy.
In earlier computer systems (pre 1980s), this kind of variety was more common. In my career, I have only been exposed to twos-complement or float formats. I would bet that this is true for many other engineers as well.
I'm glad the protobuf folks used it. I'm also glad Dr. Lemire wrote an article about it. It is another clever trick to have in the toolbox.
Think about the encoding of minus 1 for an example that differs...