I've been thinking about your question regarding the move c4 by black.
The move has useful advantages in my view.
1) By forcing the exchange of whites light squared bishop, we have a potential bishop of opposite colors ending, so black might feel like he has "draw in hand", so to speak.
2) Black forever prevents the white c3 pawn from advancing and relieving white of his chief weakness ( the doubled pawns).
3) By shutting down the Q side, black forces white to try to win on the K side. However, without the light squared bishop, white cannot challenge black's control of key squares like e4 and f5 (looking at the position after 13. ...fxg6 and especially after 34. Ne7; blacks grip on f5 is ironclad). Your point is f4-f5 isn't dangerous anyway, but black makes sure white can't force it. I imagine this frustrated white into attempting an unsound continuation in order to break through.
Uhlmann seemed to have played the game for "draw with option to win (in case of a mistake by the opponent)". Many GM's have played that way with black.
I'd welcome some French experts to weight in on what circumstances make c4 a good choice, vs. maintaining the tension!