I may be speaking out of turn here. As such, please take anything I've said with a grain of salt until Celianna or another authoritative figure is able to clear things up further.
These forums seem to specifically be about the resource packs found and sold on the
RPGMakerWeb.com site,
forums,
some of the listings on Celianna's site, and
Steam as RPG Maker VX Ace DLC. I believe the resources you are referring to, as linked per Celianna's personal site, are not apart of these listings and as such don't follow the usual terms of use provided from those purchases. Resources made by Celianna (and others) are spoken about in seemingly more appropriate sections such as
General Resources and
RPG Maker VX Ace Resources.
All that said, there does seem to be
some ambiguity with wording as to what is and is not allowed as requested by Celianna for their resources. If you view
this thread or
this one, you'll see the following.
Can be used:
Only in RPG Maker products. Meant for VX, but can be used in Ace as well!
So here we have the wording insinuating that these resources could indeed be used in RPG Maker XP without issue. The resources were linked in the VX/VX Ace resource section due to compatibility, but due to this, it does add slightly to the confusion.
Now we've seen the above quoted from the local forum threads. Here's what her website has to say on the matter.
Terms of usage:
Can only be used in RPG Maker VX and RPG Maker VX Ace, and in NO other game engine. Can not be redistributed or sold as-is. Can be edited however you want. Edits cannot be redistributed. Only for non-commercial usage.
So while the site, in wording, seems more restrictive in terms of which software is able to utilize the resources, it does explicitly state that the resources may be edited as you'd like (such as converting them for use in XP). With a combination of the two sourced terms of use, it would
appear that
you can in fact modify the resources and use them in your XP project, assuming you are abiding by all other stipulations set forward.
Take all of that as you may. I think you'd be in the clear, but it is always best to get clear confirmation from the respective owners of any materials you plan to use to save yourself from possible headaches in the future. Hopefully I've helped at least a little bit until someone more fitting can get back to you!