It sounds like you're conflating "this is a mediocre anti-tunneling perk" with "the changes BHVR made to this perk will help stop tunneling," which is not at all the same claim. New DH is a perk that will activate only when you are unhooked, with the intention being to make it harder to catch you after you've been hooked, and not before. It is by definition an anti-tunneling perk. "Anti-tunneling" is a category, not a statement of quality; if Decisive Strike stunned the killer for half a second and slowed you down for the next 30 seconds, it would still be an anti-tunneling perk, just a really, really bad one (which is not to say that DS in its current state is particularly good anyways).

Your claim that DH will promote tunneling is incorrect and relies on the idea that killers will assume that every survivor they encounter is running it. Killers absolutely do play around DH with every survivor they encounter now, because it's a problematic, overpowered perk that is far and away the best in the game and is used by 2-3 survivors every single match. New DH has a maximum of two uses while still incurring exhaustion, prohibiting or at the very least heavily limiting the use of other, more reliable exhaustion perks. Its usage is going to drop dramatically, and after a few weeks killers will (for the first time since David King came out) not have to assume that every survivor is running it until they prove otherwise.

Again, nobody is claiming that this change is going to help with the tunneling situation, but DS is an anti-tunneling perk that will see one successful use in a match at most; whether or not it is effective at protecting you from being tunneled (it isn't and the stun timer needs to be reverted), killers do not tunnel because DS exists. The possibility of a survivor running Decisive Strike is not the reason any killer alive tunnels; it does not promote tunneling, and neither will new DH. Killers occasionally hard tunnel because the game does not actively disincentivize this, which is a problem that needs to be addressed that is nonetheless not remotely a result of any perk that activates after being unhooked existing (and killers frequently prioritize survivors who are more vulnerable and have been hooked more because it's the killer's objective to do so, and survivors on this subreddit tend to conflate that with tunneling to complain about losing games fair and square).