"Every three months a Fly Quiet table will take the top 50 Heathrow airlines (by number of flights per quarter)."
Yes, it does indeed say that. To quote further from the explanatory notes:
"this aims to eliminate
skewing results by including operators with infrequent operations"
But how exactly would including carriers like Kuwait, TAM, PIA, Royal Air Maroc, Korean, etc - all of whom fly into Heathrow every day - have "skewed" the results ?
I don't see any reason why the other 30 or so scheduled operators couldn't have been listed as well.