Well, the moderators sure do know a lot, they are experienced pilgrims and I have confidence they have a lot of insight into the topic. But moderating a forum doesn't necessarily mean to know more or having definitive answers. Especially in this case, there simply is no definitive answer to the original question:
There are different albuerge owners who have different motivations. As davebugg cited, one purpose of the credencial is "access to hostels offered by the Christian hospitality of the Way". Thus, an albuerge parroquial could demand an officially issued credencial. A privately run hostel, camping site or albergue municipal would have no reason to insist on that. All of them want to sell their places, so it's highly unlikely that you get sent away anywhere due to a self-made credencial.
Antonius' question though is not focussed on being admitted at an albuerge, but if he will be seen as a "genuine pilgrim". But who would decide that? In the end, that's a religious question. A credencial of any kind won't make you a genuine pilgrim, if you don't feel like one yourself. It's your heart, your mind, your soul, your spirit and your belief which make you a pilgrim.
That's probably not the definitive answer you're seeking, but I couldn't find a shorter and equally honest one.
Buen camino
Harald