That's not really the question. The inference, is that the actual missing part of the paintbrush was found, but purposefully left out of the AR.
If you wanted to be purposefully vague (which I completely disagree with this premise, but for argument's sake), and the paintbrush was actually present, you could have used another term other than "birefringent material" to describe a chunk of wood that was serrated on both sides. You could have simply said "a foreign object." That could be anything also. But "birefringent material" was used because, it wasn't an entire piece (i.e. large part of the broken paintbrush), but rather, tiny specks.
Honestly, all one has to do is read the dang autopsy report. The "birefringent material" is listed under the section"Microscopic Description." In other words: specks that refracted light were found, which came from an unknown source.
Lastly, to put this ridiculous theory to bed: if the ME wanted to "preserve the crime scene" (whatever that means, considering the ME has absolutely nothing to do with the "scene," but rather, the actual victim), why wouldn't the ME have hidden the fact that the garrotte was made by the paintbrush as well? Why include it in one part of the AR (i.e. that it was found in the garrotte), but hide it in another part? Again, that doesn't make sense at all.