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.
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".