At the outset, he identifies three broad categories of funding: (1) luring young people into STEM careers; (2) promoting science and technology at graduate schools; and (3) raising standards of science education. But the rest of his argument lumps these all together as if all the spending is for category #1, "luring".
For example, he points out that "no such government-backed programmes exist to pull children into being lawyers or accountants." And his broad argument, as expressed in the subheading, is that the funding biases the system to producing more scientists and technologists than it needs: "If programmes to bolster STEM education are effective, they distort the labour market..."
Only by ignoring the distinction between category #1, "luring", and category #3, "raising standards", can he complain that businesses who want better-educated scientists, if they really meant it, would just pony up and "pay and train newly graduated scientists and engineers properly." Either he thinks that the technical skills businesses want can be achieved by a quick training session after a broad general education, or else he thinks that raising the salaries of engineers will send a market signal to grade-schoolers that they ought to be more interested in STEM education. The problem of poor preparation never seems to enter his head. If all the accounting firms in the country were complaining that few recent college grads can add a column of numbers, and the government responded by trying to improve arithmetic education, would Macilwain complain about the money spent "luring children into accounting"?
In short, he might have an argument. But until he grounds it in specifics, he's just hand-waving with the broad idea that trying to improve STEM education distorts the market. This is obvious tripe, because the same broad argument could be applied to education in general: "If programs to bolster education are effective, they distort the market for educated workers. If they aren't, they're a waste of money. Besides, if businesses really wanted educated workers, they would pay and train new hires properly."