It may not even be a mistranslation; we still don't know how haki works on a very granular level in regards to devil fruits.
We know that CoA haki lets you "hit the real body" of logia users. We also know, from when Smoker talked to Tashigi about Law's powers, haki could be used to defend against being sliced up by Law's fruit.
We don't know mechanics like, if you coated your fist in CoA haki and tried to hit a logia who was also using haki, if you would still hit the real body, or if there was some "haki power level" dynamic that allowed him to behave as a normal logia if his haki was stronger than yours.
You'll note here:
http://www.mangapanda.com/103-47005-7/one-piece/chapter-574.html
Akainu mentions both Marco and Vista are haki users yet he is perfectly fine and there is no blood (not saying there couldn't have been blood and it wasn't shown, just that it wasn't shown).
And then here:
http://www.mangapanda.com/103-47298-4/one-piece/chapter-575.html
Akainu is fine, but here:
http://www.mangapanda.com/103-47298-5/one-piece/chapter-575.html
Akainu is fucked up and bleeding a shitton. It would make sense that Akainu's own haki was stronger thana Marco / Vista, so he was able to brush off their haki infused attacks, but his haki wasn't as stronger as WB's, so he got owned.
This could simply mean that DD used CoA, Luffy used CoA on himself, and because Luffy's haki is stronger, his rubber powers were able to be maintained in spite of DD's CoA.
EDIT:
I also want to clarify exactly what you guys are talking about.
http://readms.com/r/one_piece/784/2764/20
http://www.mangapanda.com/one-piece/784/15
In the mangastream translation, it says "Even though
he's hardened by Haki, he's still rubbery?!" and the mangapanda translation says "He's still rubber,
in armament?!"
Those imply DD is talking about Luffy's
own haki coating keeping his rubber status, not DD's Haki coated attack (he is hardened, not I am hardened, and in armament, not against armament)
I'm confused because Chou says
which implies chou is talking about Luffy maintaining rubber in spite of
DD's haki, but the translations have DD being surprised Luffy is still rubber in spite of
Luffy's haki.