of course the seat tube length affects the geometry. so does wheelbase, bb height, fork rake and trail and h/t s/t angles.

but if you're a little off on s/t length, you can move the seatpost up or down quite a bit and adjust the fore-aft position of the saddle to get the right positioning on the bike. creating the exact correct contact points without affecting your pedal stroke.

if your top tube's off, all you can do to get the right contact points is get a long or short stem. and that does affect steering.

long seatpost = ugly, short stem = compromised steering

pick based on your priorities.