This is incorrect: this is neither what the Regulation says nor what the CJEU has found. I refer you, in particular, the Finnair v Lassooy case, in which the CJEU found that "the EU legislature expanded the scope of the definition of ‘denied boarding’ beyond merely situations where boarding is denied on account of overbooking referred to previously in Article 1 of Regulation No 295/91, and construed ‘denied boarding’ broadly as covering all circumstances in which an air carrier might refuse to carry a passenger."
It is indeed, hence why I took care to specify that compensation would only be due if the OP presented himself with adequate travel documents.
There is no basis whatsoever in either the text of the regulation or the caselaw of the CJEU for a defence of "reasonable belief" that the passenger should be denied boarding, even if one assumes that an erroneous belief of this kind by a professional could be regarded as "reasonable."