Based upon experience and professional observations (you know, all the experts at FR), this line raises too many flags. Unless the writer knows what the police know, there is no way to prove that skill of the shooter.
I agree with other comments along the lines of ... the shooter knows enough to be dangerous, and may have some skills, but is NOT an expert. I have not seen a report that suggests the shooter is lining up 500 yards away. One shot, one kill is indeed a snipers creed. Not even the police suggest that they are dealing with a professional sniper (say ex-Spetnaz or a true military sniper).
The author of this email is correct in many areas, but when the DC police find the shooter I don't think anyone will confuse him with a real pro.
A real pro would have a 10/10 kill record along with 10/10 getaways.
I would imagine this makes the sniper a gifted amateur on the presumption that 8/10 and 10/10 is beyond the probability that the casual serial killer with this degree of boldness would get.
One outlier that may be worth examining more closely is the lady who was wounded in the back. Not only did she survive, but according to reports she was released from the hospital. What is different with this case? In particular, can it be ascertained that the sniper was far away when the shot was fired, and the bullet lost energy before it hit, or was it simple bad aim from close up? The former might yield the "500 yard" conjecture; the latter, a distinct temporary lapse in skill on the part of the shooter, UNLESS there is more than one sniper...