You have just hit upon the problem with using old dictionaries. Pretentious has not had either of those meanings in a very long time, and though the semantic drift of a word may originate in misuse or metaphor, once the new meaning is established in common use, it becomes the meaning of the word. I suppose I could use "silly" to mean "blessed" or "censure" to mean "evaluate" in writing today, but I wouldn't expect many readers to understand those words used that way. What, pray, makes language change preserved by the accident of print in the 18th century more valid than the language change that has occurred since?