There are multiple problems with this perspective. First, trainers with superficial knowledge tend to teach half-truths and myths without being aware they are doing so. Second, such trainers can rapidly become unbelievable when they cannot respond to questions in seminars that require more advanced knowledge. Third, if a trainer can learn about a topic from a book, so can your staff, or at least to the same superficial level. Look for trainers that specialize in the topic area you are interested in.
Nobody can be expert at everything, or even many things.
Ask trainers about where they learned about the topic they are teaching. Consider asking for specific references that they have used to build the seminar. Knowledgable, expert trainers will probably be able to give you multiple references, and names of people they have taken ideas from. Less knowledgable people will tend to give you only one name, or one perspective. Ask trainers if there is any kind of training they will not deliver.