I hate to try and speak for someone else (quite dangerous to do so!) but I "think" he's referring to a general, accumulating snowfall, and not just "flakes in the air." I mean, the May snow days were just 2 years ago, so I don't think he's ignoring that. He's simply saying that the chances of snow falling and accumulating in late April are exceptionally rare. In fact, the only "snowfall" I remember falling during April was on 4/6/1982. That event was probably as anomalous as one can imagine...a noon temp of 26 with heavy snow and wind and snow accumulating, blowing, and drifting on every surface imaginable. The only other April snows that I can recall down this way occurred at night and/or did not stick on paved surfaces.
Flakes flying in the air in late April? Sure. A general snowfall that accumulates during the day? Can't recall that happening.