This is probably to sciency for you pack animals but here goes.
Not necessarily. THC testing on oral fluid is questionable at best.
Take other drugs like methamphetamine / opiates. These can realtively freely move from blood to oral fluid, but not vice versa because they get "ion locked" in oral fluid, ie these are "basic" (alkaline) drugs, which will more readily dissolve into oral fluid due to a change in pH from blood to oral fluid. These drugs once protonated can't passively return to the bloodstream, so end up concentrating into oral fluid. Generally speaking, using meth as an example, the oral fluid concentrations are up to 10x higher than in blood / plasma.
THC however is an acidic drug, as well as being quite non polar, so if given the choice, it'll remain in blood where it'll happily bind to proteins etc. There is some transfer to oral fluid, but you'll find that the oral fluid THC level is much lower than the blood / plasma level.
Because it's an acidic drug, this "ion locking" doesn't occur, because it's already protonated (being acidic).
That being said, you can test positive on an oral fluid swab for THC, but this is mostly from the residual THC in the oral cavity that "sticks" there because it's ionised when smoked. This "sticking" also contaminates your buccal glands which produce saliva. Eating or using an expectorant to "empty" the contaminated oral fluid from these glands means hardly any THC remains in the oral cavity.
Now, eating cannabis avoids the process of ionising the molecule because it's not being smoked, and while yes, eating it will contaminate the oral cavity, it's not in the same way that smoking does.
A number of studies have found that although when eating cannabis you tend to consume much more THC than if U smoke a joint (if the brownies are good), the respective oral fluid THC levels are significantly lower than if it is smoked.
However, even if only 0.1% of systemic THC transfers into the oral cavity, you'll test positive.
The other thing to remember is that eating THC takes longer to take effect, but lasts significantly longer. So if you're going to test positive, the "risky" time for testing is much longer when eating it.