He also later added that this was not just applicable to Timeless Isle bosses, but to all of them. So presumably, that's every boss from Galleon and the Sha of Anger, through Nalak and Oondasta, right to the Celestials, and Ordos.
What this means is just as explained above -- if the opposite faction pulls a boss, you can punch it and still get credit and loot. You'd have to be a little brave or foolhardy to do so alone on a PvP server, unless you're a stealth class, but the opportunity remains. It'll be interesting to see whether this change has an impact on the use of the Raid Finder and oQueue for Celestials groups, or not. Given the convenience of those methods for finding a composed group, I suspect they'll prevail.