I disagree with this. And the rules are pretty straightforward with that.
In opposed rolls, you cancel opponent successes. Each one of yours cancels one of your opponents, thus the one that rolled at least one more succeeds in whatever they were trying to do.
When blocking, you specifically cancel damage. You already have been hit. There's no opposed roll to avoid being hit. Either your attacker succeeds in the attack roll and deals damage plus stunts, or not. Thus, blocking is just a reflexive action to lower the damage. And also, that's why the 'damage' value of the attack can make blocking more difficult (Ex: Headbites vs 'regular' attacks).
Edit: Also, consider this: For many Xeno attacks, there's absolutely no reason for increasing the 'damage' of the attack, since the attack will outright kill you with a fixed critical result if it succeeds, regardless of how much numerical damage the attack really did. But, you may have already guessed it... yes... The only reason is to make the attack more difficult to block.
Not entirely sure with what you disagree on. But it explicitly says in the rules re: Blocking "For each [success] you roll,
choose an effect below:" and then there's a list of actions the blocking party can do. The first one is headlined as "Decrease Damage" but in the description it says that you remove a [success] from the attacker for every [success] you spend. Since the attacker chooses their stunts at a later point -- one being increasing the damage -- it could be what the headline says, but it could also stop the attacker from achieving other positive outcomes such as you (the defender) losing your weapon or throwing you to the ground.
But it's important to note that blocking doesn't remove damage "points" in itself. If you successfully remove all the attackers successes your own last blocking success removes the Base Damage of the weapon which can be 1 or 3 or something else. If the Base Damage is 3, you do not need to spend 3 successes to lower this damage, you just need to remove all the attackers successes with your own.
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Armour on the other hand does. The armour roll is made after damage has been decided and for each success on the armour roll you remove one point of damage.
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As for Xeno attacks, here the best course of action for someone being attacked is to lower the damage as much as possible if possible as some attacks cause certain critical injuries. But the critical injury only happens if the attack causes damage so it follows the same rules as above.
Hope this helps, and please let me know if I've misunderstood anything.