Re: Wait? I'm a contractior now?
That's why there are several different factors to take into account. If substitution is permitted, that may be enough by itself, but it's not the only factor.
FWIW, your hypothetical architect is probably neither an employee or a contractor of the client: they'll be working through an architectural practice, which may be a partnership, or may employ them. If they were really independent, maybe they'd have a personal services company.
In general, what matters is whether something is a genuine difference or just a form of words. If you set up a PSC to avoid the unlimited personal liability that comes with being self employed, then there's a real transfer of risk, and the two positions are distinguishable as a result. If your self employment didn't have any risk attached, you'd find it much harder to justify using a PSC.