I know that Fangraph's has their definition of 'high leverage' but I'm not at all sure it pertains to the topics of clutch and choke. Only the player can determine whether he feels that he's in a 'high leverage situation' and it's how a player responds to that feeling that determines clutch.. or choke, for that matter.
Besides,not all 'high leverage' situations are created equal. For example, a rookie player who's 0-for-22 and knows he's on the verge of being sent down may think each at- bat is a 'high leverage' situation because... for him - it is! Does Fangraphs take that into consideration?
There are also different degrees of 'high leverage'. If David Ortiz is at bat in the 9th inning with a runner on base and behind by a run in the regular season as 'high leverage' as Papi in that same situation if it's the 7th game of the WS?
I know that 'they' are trying to quantify everything but IMHO there are some things, some parts of a game, that just aren't quantifiable because they deal with a player's emotions and neither Fangraphs or anyone else knows what's going through that player's mind.