OpenWrt works for some modems pretty fine. It's not straight forward as the VDSL firmware can't often be distributed but poeple use it on avm Fritzbox devices. Also LTE devices are supported. Not sure about cable modems, probably not. It's probably involved and not straight forward so for most users, even technical ones it's no alternative.

The article says that the "modems" affected are the Sagemcom F5380 and ActionTec T3200 which from a quick search looks like full fledged CPEs aka routers with a web interface and NAT, WIFI and all the stuff. They also write about Censys and banners so it looks like they had their web interface exposed to the Internet.

When I hear people say they use OpenWRT I assume they have their modem in bridge mode so that it doesn't even have an IP. OpenWRT would save you in that case.

Ah, that's what I get for not reading the article.