not quite
1 million thresh games have been played within that last month, 20% of them choose that skill order, therefor, 200,000 games have been played with that skill combination
now, if you divide that by 10 (as i'm sure most people get tired after 10 games of league a day, we could round it up to 12 as lets say a generous "estimate" of high-profile players...let's see what we get)
200,000 / 12 = 16,666 plays of thresh, and the math gets much more complicated if you figure in monthly division as somehow I doubt there are only 500( x5 for the 20% division for a total of 2500) players playing thresh each month with that level of skill
but simply put, if you divide that 200,000 by 30, you come to 6,666 ,plays per day of high skill level, if you divide that by 12 games a day for dedicated players, you get 555 players minimum a month playing with exceptional skill level and 2777 players total minimum for just playing thresh that month
if you want evidence, here is a basic chart of probability of error for participant amounts in a survey
http://www.sciencebuddies.org/science-fair-projects/project_ideas/Soc_participants.shtml
with 2777 players minimum a month, the sample size is sufficient as to a 97.8% chance of being an accurate representation of data or if you want the actual number and not just a represenation of the chart, the chance of the data being correct is 98.102367716779921207009463303947 percent
20% pick rate is a large percentage of thresh players, one in 5 to be exact, so as I said, 1 in 20 games will have a thresh player of above average skill capable of winning 60% of his/her games with that champion...