I suspect if you look in the dungeon guide, the chest would be listed under both the prot and ret tabs, showing that Blizzard intended it to be for both. The fact that it has strength as the socket bonus doesn't mean much. There are chests with dodge that have strength bonuses; strength is ok for plate tanks, as it gives almost as much parry as parry plus it helps your dps.
Given that it's a new tier, a 522 chest is likely to be a big upgrade for a tank even if it has half decent stats and I think all plate tanks (not just palatanks) would regard mastery/hit as very nice stats.
Whether it's a bigger upgrade for a ret, I don't know. If it was a guild run, the master looter might take a view on that - depending on what the needs of the raid group were perceived to be; a beefier tank or more dps. But I don't think the tank was being disingenuous or selfish by asking for it.