Sorry to be technical, but if you analyze the sentence you'll notice the word "additional" in "increased by an additional 15%." If we were to take out the word "additional" (and the "an" to make it grammatically correct), you'd be left with "increased by 15%," which consequently is incorrect. However, the word "additional" actually is correct in this case because the change provides an additional 15% on top of the original 10% modifier. It provides a 13.6% increase over the state of healing before it, but that's not what the blues said.
Again I apologize for being technical, but calling someone out when they were technically correct just rubs me in the wrong way. No offense to you, really.

I think the key word you're missing in the original statement is additional.

Multiplicatively, you're absolutely correct.

Sterb's right; nobody really compares a resto shaman's healing versus non-resto shaman healing. We're comparing resto shaman from before the hotfix to after, and as Sterb pointed out, it's a 13.6% buff, since 1.1 * 1.136 = 1.25

The blues didn't say anything about a percentage increase, of course. They've (intermittently) learned that lesson; they just mentioned the old and new values.

The blues didn't say 15% once. Joe did. Like I wrote, the real impact of the change is a 13.6% increase. The fact is that the baseline modifier was 1.1, and as such, all future changes should be compared to it, or the modifier that preceded it.

Otherwise, what are we comparing to? Elemental healing strength?

I'm taking issue with the fact that referring to the modifier change the way it is isn't communicating useful information. What if the base modifier was 1.85? Would describing the change in terms of 15% be anything meaningful when in reality it was actually 8% change over the former state of the game?

Discussing the modifier change in terms of base healing is pointless. Context matters.

Wouldn't it just be a 1.25 multiplier instead of the 1.1?

Where Purification affected our heals by 10% before, it now has been increased by 15% and increases our heals by 25. Joe is comparing the previous effect of Purification on our heals to its current effect. NOT our old heal amounts to our new ones (Which, yes, is less than 15%) He is identifying what the buff is (not how much)
It is possibly misleading, but certainly not incorrect.