The key phrase is 'infinite extension upwards'. It's a technical detail, in practice no one would call it an obstruction except the extra-logical type but the arm is still in that space when the ball is past the peak. And it is a common approach to serving, many people are much worse in pushing the limits of it to the point that even when the arm never blocks, it is very distracting.
Interesting, I always interpreted the rule to mean that the arm had to be gotten out of the way before the ball fell to that height: that the ball had to be visible, and there had to be a clear view from the ball, wherever it is, and the vision of the returner (or the net): that, if the ball is 4 feet high and the ball is 15 feet high, or 8 feet high, then the arm cannot be in between the ball and the vision of the receiver or the net.
Also, to toss high, there is no way you can remove the arm ASAP because you have to use that arm to toss. I thought you just had to get the arm out of the way before the ball falls back down to the height of the ball. I thought it was to counteract how I was taught to serve in the 1990s when you purposely left the arm there, and the shoulder there, reaching, so that you could serve just under the shoulder, armpit and arm:
Like in the stills I am attaching:
In the 1990s people were taught to serve like that.
Isn't the rule to make it so the arm does not block the view of the ball? That is what I always understood the rule to be about.
But it is interesting that we are interpreting that term "infinite extension upwards" in opposite ways. You are interpreting it to mean, if you projected a plain upwards from the arm and that plane was between the ball and the net, that is against the rules. I guess, reading it, that might make sense from the wording.
The problem I would say with that reading is, it seems to me impossible, since the arm is actually throwing the ball, for it to never be in that plane since it starts there as the ball is on the palm and it remains there until you pull the arm back. Which would mean, by virtue of this, all serves where the ball is tossed, are against the rules and the rules also state that the ball must be tossed.
Okay. I am not sure any of this matters.