I'm going to guess most of those people couldn't set up and scale a Hadoop cluster. Are they smart enough that they could learn this stuff? Sure! But there's still a skill mismatch here.
So, obviously a LinkedIn search for exact title of "data engineer" isn't exhaustive. And as I understand your point, there's certainly no agreed upon group of skills/certification that qualify someone to be a data engineer (or data scientist, or software engineer, for that matter).
But the GP was particularly amusing to me because of its assertion that 'smart, quantitative people, regardless of industry, can build data infrastructures for startups.' I guess we could also say, there's little incentive to pay to train them (or for them to pay to train) to become a data engineer.
And I would bet across all of the tech workers in the USA there are well more than 6k that could do this.