The reason why directionals go so much better upwind - is not because of the fins (only to a very small extent), but because of its name "directional".
Meaning - it is meant to ride one way, so the rockerline is very flat at the tail and often quite long (even with round edges), and then you got scoop at the nose (and no fins) so it wont catch the water.
Way more efficient with this rockerline (heavy noselift and flat at the tail).
It can be ridden more flat - because it has more volume, and because of the flat rocker at the tail.
Meaning - it gets on a plane earlier - and you dont need so much power (or speed) to get on a plane - thus you will rock upwind really fast as you can glide with the board more flat at a high angle to the wind, and you wont lose your "kitepower" just to keep planing - but convert the power to upwind instead.
Of course - if a waveboard has a lot of tailkick, and very round edges - many flat straight TT's can go upwind just as well, easily
(although not as pleasant in chop or waves).
But the very idea of a directional, is that it is NOT symmetrical - so flat rocker at the tail, and lift and no fins at the nose - this is by far the most efficient shape by all means
Because of its bigger size and volume, somewhat bigger fins than on a TT are used, to give the same control.
Then some directionals go to the extreme - having straight sharp edges and really flat at the tail - so even with small fins they ROCK upwind, and gets on a plane ridiculously early
And for going a tad more upwind, really big fins are used - but this is mostly for competition where even the slightest higher angle can decide who wins
So all the good performances of directionals are simply because they ARE "directionals"