I think it's about what exactly is the DMCA used against. My first thought was as well that they claimed the act of writing down the domain name somehow violated the copyright of their domain. That sounds kind of silly and if it were actually the case, I think a hash/regex solution would make sense.

However I think what actually happened was that the business is operating paywalls/"anti-adblocker-walls" for other sites - so they claim that blocking them constitutes "circumvention of protection devices" for their customers - which indeed would be far severe for adblockers if confirmed by a judge.

(That's my understanding, though I might have gotten it wrong)