Good point. However, if you don't clear and hidden focal infections (like infected tonsils, ileoceacal valve, dead teeth, etc.) then your immune system can't be sufficiently "powered up" to kick a yeast infection, regardless where it resides in the body, whether it be in the gut, sinus cavities, vagina, etc.
The "impossible" yeast infection cases may well have such an obstacle, and i have seen this in about 5% of cases.
The point you have made is quite valid and unfortunately overlooked - yeast infections cannot clear satisfactorily unless the GI tract is in balance.