The most charitable interpretation I can think of to this claim about privilege is that the FBI seized material so indiscriminately from Cohen that it may contain things that do genuinely fall under the ambit of privileged information. This assumes, of course,

• That the FBI indeed seized material indiscriminately and not just incriminating documents that they had such extraordinarily strong evidence of existence that they were able to convince several of Trump's own DOJ appointees of the importance of a search warrant
• That Trump and Cohen have a bona fide professional relationship, and not that Cohen is just Trump's fixer
• That Cohen is a totally legit guy

Among many other such preposterous assumptions.