They could do what modern computers do and apply error correction codes. I mean if we're already teleporting knowledge into the past we might as well go all the way.
There are several slight distinctions to be made there. A keynote/powerpoint presentation is typically displayed as though on a piece of paper (A4, say), and thus of course type should be rendered proportionate to the virtual piece of paper, no matter the display. The post is referring mostly to HTML sizes.
The second distinction is that a projector or video monitor should duplicate the master screen, and indeed for a projector you have no idea how big the final screen is, and can change its size by moving the projector around. Video projections, similar to paper, are a proportionate medium.
Not making these distinctions is definitely an oversight of the linked article. His major point is well illustrated in his example picture, where "5 inches" has been displayed with varying degrees of accuracy, but it's clear all the monitors are trying to display something _close_ to 5 inches. Not getting it exact (±½ pixel) is a failure.