No, clerics do not have their full spell list prepared—it’s not even clear what that would look like in Pathfinder, where spell preparation requires not only choosing spells, but also how many copies of each. A statement like that really only makes sense when discussing a scheme like D&D 5e, where you choose spells to prepare but cast “spontaneously” among the spells prepared—though of course, the 5e cleric does not have all of its spells prepared, since that would defeat the purpose of preparing them. I suspect either you or the person you were reading was mistaken about which game was under discussion.
A cleric is limited in the number of spells they can prepare by their spells per day—even if they prepare only one copy of each spell, they soon run out of spell slots in which to prepare spells and there are always going to be many, many spells on the cleric spell list that they haven’t been able to prepare. However, a cleric can choose among all cleric spells when choosing which spells to prepare—they don’t have any mechanism limiting their options when preparing spells, like wizards do. It’s plausible that the person you were reading was misusing the word “prepared” to refer to “spells they can prepare”—which are, indeed, the contents of the full cleric spell list (barring alignment conflicts).
Either way, whatever you read was wrong; clerics definitely cannot have every spell prepared, much less always do by default.