Yes, this is scam prevention 101. Anyone who called you is always unverified. It's hard for me to take seriously a "scam prevention expert" who doesn't seem to know or follow this rule, which by itself is enough to protect you from most scams. Normally I try not to victim blame people for getting scammed, but when you've made a declaration like that you forfeit that right.
I'll also point out that the author seems to have some complicated arrangement for their phone number(s), presumably in the name of security, that in fact got in the way of identifying this to be a scam.
Regarding the complex phone arrangement: There's an effect, the name escapes me, that adding security can make threats less frequent but more dangerous. Sounds like he was more complacent because he had trust in his phone system.
And I agree about author - if he had said that he violated an easy rule and owned that I would take his credentials more seriously. Everyone makes mistakes, but he didn't list this simple, well-known rule as a way of preventing this.
I can see a normal person falling for this, but in my opinion this person called themselves a scam expert is a scam in itself. The claim that this has only been praise since 2018 is absurd, even if true being four years behind on current practices is making you a no longer expert.
I'll also point out that the author seems to have some complicated arrangement for their phone number(s), presumably in the name of security, that in fact got in the way of identifying this to be a scam.