This claim seems to be predicated on some unrealistic standard wherein all businesses have dedicated professional in SEO and or advertisement management (or outsourcing to media management companies who do).

That exists of course, but sole proprietors, small businesses, and medium businesses also purchase online ads in high volume and therefore people from other domains are commonly buying ads (or more importantly for this discussion: deciding where to spend limited ad dollars).

Isn't it clear the moment you buy your first ad on Facebook that there are extremely specific categories you can target, i.e. to any "person buying ads"?

Yes, non-ad people doing ad work may not be aware of it. Anyone who is an "ad person" who is unaware of the degree to which internet ads can be targeted is not an "ad person", though.