It's probably very hard to measure truffle mycelium growth: its fragile and buried under very heterogenous dirt. And mycelium is the form this organism takes the majority of the time: the fruit is just the end product.
It'd be like trying to understand how plants grow without being able to see or measure light.
According to that article, we know this protein exists, but we have never actually been able to isolate and study it directly - we just see some derived substances. Fascinating how such basic things are still somewhat mysterious; and how it's easier to create antibodies to detect byproducts of a protein than it is to chemically isolate that protein!
It'd be like trying to understand how plants grow without being able to see or measure light.