"Solo Founder" seems like a proxy that just acknowledges that Software Development is now a Profession.
We don't refer to Doctors, Lawyers, and Accountants as "Solo Doctors", etc...
They are simply people who have studied their area of expertise for years, are disciplined and principled in their approach, likely carry some form of E&O insurance to warrant their work, and are perfectly capable forming their own legal entity and opening their own "office".
Broadly speaking, these professionals belong to networks of other professionals, who occasionally collaborate on projects.
Reading the article in this context then, focusing on a niche makes perfect sense for any Professional.
There's a difference between working a job and being an entrepreneur. The solo founder vs partnership debate comes up in every profession you mentioned, e.g. when starting your own law practice or medical clinic from scratch. Particularly for lawyers the standard advice is to at minimum have a group of partners with complementary skills (deep technical knowledge of laws, experience in a courtroom, ability to schmooze clients), not very different from what you'd hear from a tech VC.
We don't refer to Doctors, Lawyers, and Accountants as "Solo Doctors", etc...
They are simply people who have studied their area of expertise for years, are disciplined and principled in their approach, likely carry some form of E&O insurance to warrant their work, and are perfectly capable forming their own legal entity and opening their own "office".
Broadly speaking, these professionals belong to networks of other professionals, who occasionally collaborate on projects.
Reading the article in this context then, focusing on a niche makes perfect sense for any Professional.