The author says that a constant lookup table is extra implementation complexity while also saying it's simple and something every programmer should have done as an educational exercise. Considering the density of browser specifications, talking about symbolic constants as feature bloat is some serious windmill-tilting.
They then say that named constants make code harder to understand -- which is obviously an opinion, but one that goes against almost all standard practice. The benefits of naming constants and avoiding magic numbers are well documented, I think. Yes people can go overboard with this, but the solution isn't to not have named constants as an option at all.
The kicker I think is the sheer popularity of CSS preprocessors. When people are compiling to something you designed as an end-user interface, you've failed.
The problem with CSS variables isn't the concept, its that --this-syntax is ugly and unreadable af (admittedly bikeshedding here).