Solo founder is such a loaded term. Every freelancer, dropshipper, consultant is also a solo-founder. it's just not as glamorous as building a venture backed product.
No, a solo founder is someone who starts a new product business. That is very different from consulting, freelancing, etc, where you sell your time.
Did you just make that rule up?
There's lots of bullshit on both sides. "Building a venture-backed product" these days often means "being a full-time salesperson peddling grossly overvalued equity by trying to schmooze people who don't understand the underlying tech because that's the only way for the existing investors to exit".