That issue isn’t a dnssec problem, it’s that Let’s Encrypt was not familiar with the route hijacking threat model. It was pointed out early to them and they ignored it.
I run a network, we do the whole shabang of RPKI, DNSSEC, and CAA. It sounds a whole lot like operators who refuse to address clear security issues. LetsEncrypt is not to blame when someone spoofs your address space.
LetsEncrypt is not a LIR/RIR, their business is not IP resources but SSL certificates. They are a CA. They have no tools available to them to address that problem.