I appreciate the links but I think this actually misses the point. The novelty isn't in diagnosis of cancer but in sensitivity/cost-efficacy in detection of known biomarkers for breast cancer (and associated risks of recurrence, etc). I'm not familiar with how common ELISA-based HER2 testing takes place, but it seems like it has some impact on drug decisions[^1].
In terms of applicability, it depends on whether or not ELISA is in fact the current standard of care, but it could be useful in low-resource settings where you don't have lab personnel trained to carry out those assays, and drug choice is also restricted by limited availability.
Additionally, there's a point-of-care argument as well. Since breast cancer does benefit from early detection, I can see a future in which biomarker testing is a more regular thing, and high saliva concentrations are flagged. At the very least as something worth bringing up at one's next appointment or wtv.