I know and I agree. But when people make such an argument about goalscoring also being bound by some of those specifics you mention (and many others), it receives far less understanding.
Assists and throughballs, etc. are dependent on dozens of factors apart from the player himself, but I guess goals are goals nothing more, eh? Go figure.
On a sidenote, I don't think the examples you gave "to aim at" are very good ones. Iniesta almost never makes those off the ball runs, Pedro and Sánchez do it very infrequently as well in the last two seasons anyway (Pedro and Villa 2010-12 made loads of those runs, hence Messi's TB per game stat being so high in those seasons). Neymar this season has at times provided a decent "target" but still nothing close to what, say, a Walcott does at Arsenal, or, if you want to stay in the Messi context, what Tello does.