Defender (almost) certainly was illegally positioned before the throw if they are able to block it in this manner, but 18.1.6 places the responsibility on the thrower to 1. notice and call the defender’s illegal positioning and 2. not throw the disc. This is not the only rule working this way, generally if offense doesn’t notice a breach and plays on, there is no recourse short of 1.2.1.
The important difference between all of the explicit rules and the catch-all (logical bottom) 1.2.1 is that any change in the outcome requires the agreement of both captains, whereas normally disagreement between the teams always sends the disc back.
The opposite rule (logical top) 16.3 (play stands if all parties agree there was no effect) does not have this flaw.