Quote from that page (underlined by me):
"Note: This version of Xarchiver has been written for the previous version of the Maemo platform (Diablo) and hasn't been ported to Maemo 5 yet. However, since it works just fine on the N900, for your convenience we are making it available via our own repository. Other than not being optified (i.e. installing in rootfs and taking some space there rather than in the large opt partition) there are no other issues, and it makes handling archives much more convenient than using command line based tools."

(The home page http://xarchiver.xfce.org/ (shown in xarchive.garage.maemo.org) doesn't work now.)

MohammadAG has just made a proper port and optified it. Good work. Thanks.