I don't think you're nuts, but I do disagree slightly for a number of reasons. For one thing, I have seen Federer hit stab volleys with pace before very effectively. Its not something he regularly does, but he is capable of doing it. But the fact is, he's a different kind of volleyer than say Sampras, who was known to power his way to the net and hit a volley winner off the second or third shot. Mahut has the same mindset as Sampras (only he's not as good). Federer rarely serves and volleys, but rather takes the ball early and hits sharply angled approach shots to make his way to the net. His athleticism is also different from a Sampras, who was more explosive front to back, and up and down. Federer is more graceful moving diagonally, side to side, and in weaving his way forward. So the position he's often in when he hits a volley is different.
Also, something that often gets lost when discussing the quality of volleys among different players is the quality of passing shots they face respectfully. And that is largely influenced by the quality and timeliness of the approach shots by the volleyer. When Sampras played Agassi, one of 4 things was going to happen on his service games. Either Sampras was going to hit an ace, Agassi was going to hit a return winner, Sampras was going to hit a good enough serve to elicit a weak return, or Agassi would hit a good enough return to make the rally neutral. Only the third scenario would put Sampras in a position to hit a volley, and the best volley to hit in that scenario would be a hard, stick volley winner.
Federer more often than not will hit a soft volley (either sliced, angles, or drop volley) because the shots that he chooses to come in on necessitate that type of volley. When Federer is coming to net (usually on his own terms) it's usually not immediately after the serve. He's usually very strategic about where he places the volley. When Sampras came to net, the point was going to end one way or the other. Federer doesn't need to hit firm stab volleys with pace in order to end the point because his placement is so good. He is also willing to restart the point and get back into the rally in case the net approach fails. Sampras was not willing to do that. That doesn't mean Federer isn't as good of a volleyer, it's means he has a different mindset when he's at the net.