Mark, as you would know, open fermenters have very high headspace, as at Samuel Smith:
When the vigorous phase is done, the beer goes into casks or secondary fermenters with little headspace. Similarly, it's only a concern for home brewers if they leave the beer in primary until bottling or kegging.
Even then, 30, 40, 50 or even 60 percent headspace should be little worry so long as the fermenter is airtight (If it's leaky, a small headspace won't help much). To illustrate, fermentation of beer to 5.75% abv (4.6% alcohol by weight) produces one mole, or 44g of carbon dioxide, 22.4L at standard temperature and pressure. In a half-full 30L fermenter that would amount to 336L, of which maybe 10-15L would be retained in the beer, depending on temperature. Ten times the volume of the headspace. That's why the trap bubbles so much. There is a complication, in that the heavier carbon dioxide both displaces and dilutes the air in the headspace, but not to where substantial oxygen would remain in this example. I usually ferment 19-20L in a 30L fermenter.
Where the 20% rule came from, I have no idea, but when I see a post that a high-alcohol beer, which produces proportionately more carbon dioxide, has a tiny headspace and a krausen monster is overrunning the kitchen or mancave, it's no surprise..