To answer your question, (though Luna, Shaz and Sharm already answered it before): Yes, you need to own XP to use those sprites in Ace. Why? Because they are edits of the XP RTP and as mentioned before, when you buy the program you also get the license to use its RTP in other RPG Makers/IG.
Maybe it'd help to up the price of GCH to include the price of RMXP (and bundle RMXP with it). And those who already have RMXP and don't want to pay the high cost can submit a support ticket to substantiate proof of XP ownership and get the GCH by itself.
Or even have it bold print on the main page. I've searched Steam and another website, and nowhere does it state that owning a license to RMXP is required before using this (unless I'm blind), so you can potentially add it to your cart and pay for it before you realize you may not be able to use it.
This is probably a non-issue for most who either already own RMXP or purchased it during the bundle (just having access to the audio was incentive enough to buy), but for others it may feel like a scam to buy into this and then realize you need to spend an additional chunk of change just to buy the license to use the materials in there.
I am just looking through all the stuff at the moment to find something with substance. It is most likely, that the people here are right, but I want to read the passage where it is declared how resources are meant to be used, before I tell other people what they need to buy or not to buy. There was probably something I agreed to, when I first started RPG-Maker... but well ;-).
So to bring my thoughts to an end, I wrote to the support team of rpg-maker web. And got the following answer from Asley Maxwell.
"When you buy the RPG Maker program you can use the RTP program only with the program it came with, and any other RPG Maker program that you have purchased legally. It does not give you the access or rights to use previous RTPs unless you own those programs as well."
It doesn't change my opinion about how things should be, but I agree with Grandma that it's not much money for what you get (even when you pay the regular price).