More precisely, a magic domain cleric can use those items as a wizard of 1/2 level relative to his cleric class level (as it is a domain power). So, a 6th-level cleric can use the items as a 3rd-level wizard. A cleric 6/Fighter 4 with Practiced Spellcaster feat (now caster level 10 as a cleric) still use the items as a 3rd-level wizard.