> While this feature allows for some creative layering in static typesetting, it is decidedly spectacular for interface design, where things tend to quickly go awry with only small changes. As an example, changing the weight of a single menu item on hover usually results in an erratic twitching dance of the whole menu as it tries to adjust for the change.
> this feature is nothing but a nice gimmick in static typesetting,
The point is that when the font attributes change dynamically when using the interface; like hovering over a button makes the text bold. If that dynamic during-interaction change changes the size, it might make everything else move around too, which would be bad.
The problem being addressed isn't so much being able to get the page to look just so, but being able to make it not jitter around while you interact with it.
Here's the animation from the article that I'd use to get the point across (top is a non-uniwidth font, bottom is a uniwidth font): https://miro.medium.com/max/1400/1*eUu31P1t6Ez6xwD56BQJaw.gi...