The writing is a bit ambiguous, but if I'm parsing it correctly it's only a big deal to the small subset of physicists who care about mathematical rigor: proof that Quantum Field Theory (i.e. what theorists have been using since the 1940s or so) is mathematically sound.

This part,

The existence proof pertains to the simplest grand unified theory that includes the gauge fields and fermions of the Standard Model

looks like a reference to SU(5) GUT from 1974:

https://en.wikipedia.org/wiki/Georgi%E2%80%93Glashow_model