After reading I realised that this proposal isn’t a single new element for all neutron stars, but a separate new entry on the table for every individual neutron star in existence, unless there are two that happen to have the exact same number of protons which is unlikely. Sounds good to me

protons

we have assumed that Rex is comprised of a uniform nucleon fluid, with protons, neutrons and electrons in an idealised 1:8:1 ratio

This is how the author is estimating it, they are assuming 1/9th of the mass is protons. No idea how good that assumption is though, there is a source which doesn’t look the most convincing

Whoopsie. I used to assume neutron stars are made of neutrons. It turns out Big Astronomy lied to me.