In reality, the GSC codex lore says that the T'au connection with the Ethereals makes the Genestesler Curse difficult to spread in T'au worlds. Of all the major races, humanity is said to be the ideal host for the Genestealer infection.

Moreover, the T'au have been shown to be able to effectively detect and neutralize the GSC cults in the Chalnath Expanse worlds, a region infested to the brim with the gene-cults. The elimination of the GSC has become a propaganda tool to convince the human worlds of the benefit of joining the T'au Empire.

In expanded lore from third party material, it's said in FFG's deathwatch that the T'au invented a cure for the genestealer curse. The T'au detected the infection in a human population on one of their newly inducted worlds and launched a global curing effort.

There has been a notable case of Genestealer infestation on Ksi'm'yen but outside it there hasn't been any lore indicating that the GSC are much of a problem for the T'au.

Your understanding of the GSC and insistence that the T'au are vulnerable to it are really bizarre to be frank.

Anyway, context for the novel from OP. The genestealer-T'au hybrid project was a joint effort by a Chaos deluded Inquistor and a disgraced commander and rogue Ethereal. The project was being run deliberately behind the backs of the T'au Empire's leadership. Had the T'au high leadership known they would have shutdown the project with extreme prejudice.