The Inner Beast change is not a buff, it isn't even a neutral change. This is a flat out nerf on all content that isn't Twintania, and even for Twintania it is either somewhat neutral or still bad depending on crits and Berserk usage.
Now that that's out of the way, let me explain why this is true.
In the current game, Inner Beast heals me for about 2100 in a raid with no crit and not using Berserk. However, I am going from memory here and I don't have a screenshot of this. Getting one would prove bothersome, so i went and punched a dummy all by my lonesome and got an average of around 1500 healed. For this thread, I will use this number (500/1500) as it is about what I used to get with only 2 pieces of i90 and gryphonskin.
The changed Inner Beast will heal you for 100% of the amount damaged (500) and then apply a damage reduction of 20% for 6 seconds. So anytime the 20% damage reduction reduces damage by more than 200% of the damage done by Inner Beast (1000), the changed spell is going to be better. Which means any time you take 5000 damage or more over that 6 second period, a winner is you!
Looks great, there are several encounters where you will take 5000 damage in 6 seconds, even Titan will hit you that hard with Mountain buster followed by Rock Buster or an auto-attack! But here is where it starts to fall apart.
First, lets look at that 100% heal that Inner Beast still has, and think about when we're using the ability and what the likely outcome is. You want to use this ability a second or two before the big hit you're trying to counter. If this DR buff behaves like other buffs, it's not going to be instant and you'll want to have it active for a second or two before the ability you're trying to reduce. As such, you are likely to only reduce the attack you're trying for and anything immediately following, it simply doesn't last long enough to get the next attacks and since most large hits come with a wind-up, you are unlikely to get anything before it either. So, you've used Inner Beast a second before the big hit, and you've received your 500pt heal. If you're not at full health at this point, then your healers are struggling and it might be useful. However, you should already be at full health and with the way spike damage in this game works, if you're not at full health, you're probably going to die. So quite often, this heal is going to be completely wasted as overhealing. Now, if we ignore this heal, then you now have to be taking 7500 damage within that 6 seconds. Still doable. Death Sentence hits about this hard if it is completely ignored (i.e no Virus, no Eye for an Eye) and then there's usually a plumet or auto-attack afterwards. Of course almost nothing else in the game hits anywhere near that hard, but it could still be useful!
Next, let's factor in Berserk. Berserk can't be used for all Inner Beasts, but it can be used for some and when it is used it is 100% reliable. Berserk puts my solo Inner Beast up to 700ish (about a 40% increase) which would give me a 2100 heal. We now have 2100hp that needs to be made up by the 20%DR. This means I have to take 10500 damage from the big spike and whatever follows. We're now past anything that the game can throw at us except an un-de-buffed Death Sentence followed by a Plummet. For this to happen, your healers and mages have to be terrible, and whoever is driving the luck bus has to hate you. This does happen, but it is rare.
Next let's factor in a crit and then the absolute best case for the current setup, a crit while Berserk is active. Crits are entirely random, you cannot rely on them, you cannot trigger them, they are completely random and you should not balance around them. Having said that, they do happen and I will account for them. My solo Inner Beast hit for 500. A crit would bring that up to 750, so we're already past the point of even the Berserk situation above, we now need 11250 damage to break even. Now let's look at crit+berserk. We're now looking at 1050 damage done, 3150 damage healed and you now need 15450 damage done to you in 6 seconds to break even. This is impossible in the current game except for one-shot mechanics that you would still die to anyway.
Even if you pretend, and your example is full of terrible healers who don't have you at full before your big hit comes and so the Inner Beast heal gets used fully, and you're playing drunk and forget to ever use Maim or Storm's Eye, you still get 400 damage and a 1200 heal. Which means you need to be reducing a 4000 damage hit... about Titan level. Using just Maim and Storm's Eye, this is a side-grade against Twintania and a nerf against everything else in the game. Factor in Berserk and only extremely bad luck with the largest hit in the game followed by Plummet and an autoattack can compare. If it crits... then there is nothing in the game that causes close to enough damage.
You can now start telling me why I'm wrong.