So a secondary attack granted by a rage is still a use of a rage power. I don't see anything in RAW that would contradict this. You can validate this line of reasoning by asking yourself what otherwise is it? It must be an attack, and ALL attacks involve the use of a power, this is another line of reasoning which confirms it.
No, it's a rage attack. For it to be a use of a rage power, it has to, explicitly, be a specific POWER. Secondary Attacks are not, and have never been, themselves powers.
For example, if you have a ranger attack power that has Attack, a Secondary Attack, and a Tertiary Attack, you did not use three powers. You used one power, but made multiple attacks.
In the case of this, you are using ONE power, but you're making multiple rage attacks.
Attack and Power are two different game terms... an attack is an effect of a power. Using a power does not imply making an attack, and making an attack does not imply using a power.
For an example of a power that has a secondary POWER--look at warden's dailies. They give you a permanent effect that gives you access to an at-will power, complete with green title, the At-Will keyword, and all the keywords and details that come with a power.
All THR comes with is a line: "Secondary Attack." No descriptors, no power templating, nothing at all to indicate that it's a distinct power, as opposed to a secondary attack that is the same as every other power that grants a secondary attack. What is the usage-key-word of this 'secondary power'? If it IS a power, and it inherits the keywords of its father power, then it's has the Daily keyword, and you can only use it once per day--that's not right is it? Of course not.
I agree that using that secondary attack is, technically, using the power, in the sense you are continuing to use that power you've already used. It is not, however, a distinct use of that power. It is not using a new power, it's continuing to use a power you've already used. It does not trigger things that trigger off the use of a power, it does not use the 'Use a Power' action. In other words... if you have a power that has a secondary attack, you 'Use a Power' once, but you 'Attack' as many times as that one power use allows.
It WILL trigger any abilities that trigger off attacks as many times as it lets you do so. It will NOT trigger any abilities that trigger off powers, as you only have only used one power during the entire situation.
RAW, there is only one power used. RAW, powers can have secondary attacks. Attack and using a power are NOT the same game term, never have been. Nothing in essentials has changed that. That's been consistant through the entire life of the game.