That is consistant with my beliefs... Wordpress has the ability to edit pages directly... Powerstore doesn't. They took advantage of the security hole in wordpress to update the powerstore site. Any php page on that wordpress site was vulnerable because of wordpress. It appears it had nothing to do with the powerstore other than the powerstore is built with php pages and was hosted along with a wordpress site.
Anyone with wordpress and any php pages should be concerned, but without wordpress there would be no way to add code to the powerstore site.