My summary for programmers:

When you get a viral infection, immune cells make a signalling protein called a IFN-1 (Type I Interferon) cytokine, and this flips a boolean flag to True on a bunch of genes (interferon-stimulated genes or ISGs) that produce a bunch of proteins (hundreds) that control the infection. ISG15 is one of them and its role appears to be to downregulate and to limit the inflammation.

The paper title refers to a ISG15 deficiency, meaning if you are dificient in ISG15 that inflamation limitation goes away. But the paper is actually about how in people that naturally have a ISG15 deficiency, there is an always-on low level expression of some of these pro-inflamation genes. So they take that as a safe level.

The did RNA sequencing on experimental ISG15 deficient cells and from heatlhy individuals, identified the mutations, narrowed down to 10 genes (antiviral ones not inhibitors) that in combination significantly inhibited viral replication. They stuck the RNA for such genes in lipid nanoparticles such that they enter host cells, whose ribosomes happily read the RNA like a turing head reads a tape in base 20 and produce proteins encoded by these genes, similar to how the mRNA vaccine works.

So why not dose with the IFN-I directly? Three referenced papers show its "poorly tolerated with significant side effects" and all those downregulators that get expressed limit the inflammation response.

Disclaimer: IANAB (not a biologist) corrections might be due..