It easy to say that in hindsight but it wasn’t that unusual of advice in the early days of the web (which is when the GP is describing).

The web (and internet as a whole) was a very different playground to what it is now. In fact you only have to look at how trusting early internet protocols were and all the bolted on hacks we’ve needed since to see the change (eg SMTP, FTP etc all have security as an afterthought).