This is plain wrong. Some can’t, but you only need to look in Google News and Google Books to find examples of extreme adjectives being used with “very”. There’s no general rule. For example, here’s an example of “very delicious”:

Speaking about this specialised group, he said: “This extraordinary breed of farmer manages to produce food – and very delicious food indeed – in some of the harshest conditions; the weather is extreme, the soil is poor and the topography is some of the most challenging.”

The speaker was Prince Charles. If the future king of England uses a phrase, I think we can be sure it’s acceptable usage.