Earlier quoted context omitted.
You didnt cite examples where this is working. Sending them from pro to anti propaganda, assuming propaganda works, does work.
You didn't cite examples either, and I'm skeptical of the premise. If reddit started redirecting /r/the_donald links to anti-Trump propaganda, or Youtube started redirecting his own videos to anti-Trump propaganda, no matter how good that propaganda is, I don't believe you that you could change their mind. And those people aren't even radicalised.
The hardened core will probably feel justified in their beliefs by the attempt to combat them and radicalize even quicker.
But more casual group members might actually change their mind if they are confronted with outside opinions, especially if they hadn't considered them before.
So whether this kind of propaganda is effective or not essentially depends on the makeup of the targeted group. I guess you could try measuring the effect by tracking engagement of the redirected. If they are truly radicalized, they are likely to leave; but if they are more open-minded, they might stay a little longer.