Only in the loosest possible sense. He moves slightly to his left. Take away Dumfries and it's still a goal. He's not prevented anything as far as I'm concerned . As I said earlier, I can see why it's given but it's soft, imo.
So he moves then?
Let's break it down, shall we?
"a player moving from, or standing in, an offside position is in the way of an opponent and interferes with the movement of the opponent towards the ball"
Dumfries was clearly offside and clearly in the way of any path Maignan might have taken towards the ball.
Your entire argument hinges on the "interferes with the movement of the opponent" bit, because you seem to believe that a collision of some sort has to happen for one player to have interfered with the movement of another.
I would argue that you can quite obviously interfere with someone's movement from point A to point B if you are standing directly between point A and point B, regardless of whether that person actually attempts to move. People are aware that they cannot simply pass through other people like ghosts, and sometimes decide to make only minor movements, or even not to move at all, if their path is clearly obstructed.
In non-football terms, if someone decides to park across my driveway and block me in, the movement of my car has been interfered with, even if I don't actually move my car into or towards the car blocking it.
Even if we accept that movement has to occur, by your own admission, it did. There is nothing in the law about the extent or force of the movement, so to go back to the earlier point, we simply do not want officials trying to read the thoughts of goalkeepers in situations like this, especially when there's a reasonable chance their movement (or lack thereof) could have been influenced by the offside player standing directly in their way.
"this is an offside offence if it impacts on the ability of the opponent to play or challenge for the ball; if the player moves into the way of an opponent and impedes the opponent's progress (e.g blocks the opponent) the offence should be penalised under Law 12."
Dumfries standing within a yard of Maignan, between him and the path of the ball, is quite clearly impacting his ability to play or challenge for it. This is irrefutable, and there's not much more to add to this bit.
As tomaldinho pointed out, your issue actually seems to be with the law as written, because nothing in your argument is actually supported by it.
However, I'd even disagree with that, because your argument seems to want the law to incorporate subjectivity on why a goalkeeper may or may not attempt to make a save when there's a player directly preventing that, whether a goalkeeper is even aware of the offside player directly preventing them from attempting a save, whether the goalkeeper has sufficiently sorted his foot placement and balance out to attempt a save, or even if the goalkeeper is simply good enough to pull of the save required.
It's fine if you think Maignan's not getting there regardless of Dumfries, but the fact is that we simply can't know that precisely because Dumfries was there. To continually harp on about the law and still end up falling back on a subjective view of Maignan's ability as a goalkeeper to support your argument (that isn't all supported by the law you keep mentioning) is laughable.
