Every company I listed either grew from developer to developer + publisher or is still just developing. More importantly I don't think the distinction matters

Every company I listed either grew from developer to developer + publisher or is still just developing. More importantly I don't think the distinction matters as it's a natural next step if you accumulate enough success - Dropbox didn't start with a product for other businesses either. Specifically Rovio, SuperCell, King, Zynga and Mojang owe all their success to their own games released in the last decade. (Although king used to license/publish little flash games but they pivoted past that years ago).