Whatever the dictionary says must be right I suppose (that’s irony*), but in my mind’s ear, ‘poser’ was originally used with a strong London accent, as in:

“’E’s a right poser, i’n’ ‘e.”

… and it meant their ex school - mate now drives a BMW and wears a Rolex and a blonde with long legs and short skirts but has difficulty with the repayments.

Now we all use the word ‘poser’ to describe the person who tries to give an undeserved or overplayed impression of success, or of being at ease in an environment or activity not natural to him/her. A fundamental characteristic of the poser, to me, is that they are unconvincing. I don't think that need be the case with the poseur.

*Note: I gather that we always have to give warning when irony is used. I don’t know why.