Yeah, I can buy that. I also bought my xbox 360 Arcade in June 2010 after it was discounted to $150.The 360 S launched in june 2010 at 299$, so absolutely standard 360s would've been found cheaper and at that point in the console generation you have retailers discounts happen month in month out. Be it through bundles, or weekend price drops, whatever. Point being we can't compare the two by dollar sales.
But this June figure is literally a year later. How many of those $150 arcade consoles were even out by that point? Besides, the 500k number indicates that the xbox series x and series s most likely sold around 250-300k units seeing as how the xsx outsells xss version by a 4:1 margin in Japan and UK. Thats really not that high or hard to believe.