It's really not interesting to say that a major ISP doesn't have capacity for all of their customers to use their advertised bandwidth at once. That extreme just doesn't happen, and so the cost/benefit of preparing for it just isn't there. Some oversubscription is normal. And when I said typically overprovisioned, I meant relative to actual observed/projected load rather than theoretical worst-case.
For their upstream links to actually be maxed out (thus "experiencing congestion" as Hikikomori put it) with any regularity is more remarkable—suggests they screwed up their capacity planning or just don't care. I kind of expect that from Comcast but not ISPs in general.
For those links to be of varying capacity (like the 5G/Wifi networks the article mentions) would be truly surprising to me.