I am suprise the subject of wake vortex wasn't mentioned in the report.
At the time the light aircraft was flying into the rising sun although that time of year it would be quite high I can quite understand the visual view of thinking you are actually the same level if not below of a slightly lower aircraft. I have put the de-icing gear on enough times thinking I was going into cloud to find that I was actually above it to know that I would be fooled as well.
Now as to the point of going behined the aircraft believing that you are below it. C17 is big and heavy in my book and flying a small plastic pig your small and light, 6 miles to be safe. Although decending to avoid is contray to the rules of the air if you believed you were below it going close behind you would be setting yourself up for a fatal wake vortex incident if you followed the rules of the air.
Now not absolving the GA of all blame I would question his choice of route to fly through a hold of a busy field without getting a service with the airfield. But this isn't purely an issue with Mil fields, it is also an issue with civi airfields in class G where VFR pilots don't have a clue what the instrument procedures are. But technically he is perfectly legal to do this.
Although this is another issue because the Mil fields don't publish thier plates on the AIS website so civi pilots have no access to where the holds are even if they do know what they mean.
Thank goodness for TCAS. And I believe not all aircraft operating into Brize have it. Another day a different aircraft, a different outcome.