Can confirm Solus is not now nor was it ever based on any other distro.
History lesson:
"SolusOS" and Solus have two different origins.
SolusOS was a debian based distro and is dead.
Evolve OS had to have its name changed due to some trademark nonsense with the use of "OS" and was renamed to Solus taking the name of the old, unrelated, dead project, which was a distro also founded by Ikey so ownership to the name was secure.
So that explains why the package manager is called eopkg and why the distro is Solus but never SolusOS.
Bonus trivia. You can't just use "Linux" in your projects name without permission, which you have to apply for and meet certain criteria which most distros do not bother with. So it is never "Solus Linux" either.