Jayasuria's series against England was a year before the Zimbabwe series, not as close as you make it sound.. I've already conceded that prior to that point Zimbabwe should count. He also averaged 31 a year later against England.
This is clutching. Kumble and Harbhajan both played England right before they played Zimbabwe and weren't even near as successful. They played England, averaged 23 and 25; played Zimbabwe right after and averaged 18-19. Then Zimbabwe play SL after that; Jayasuria and Murali average 14 and 10, respectively, and Saqlain plays them later on that year (the very next Zimbabwean series) and averages 21, then he pretty much loses it and retires.
Zimbabwe played all these teams one after another in the same year. 5 spinners doing much much better than their records usually are in all the same short time-frame is not a coincidence. If it happened one this year, the other next year and the others even further than that, ok, I'd buy it. But not like this.
It doesn't seem you're going to concede this point to me and I don't buy your evaluation either. Either way, I, personally, don't find much of a difference to squabble over about - it doesn't affect his record to demean the perception about him as bowler. Peace.