The claim is slightly exaggerated. While the website itself isn't national reputable, the article was reviewed by a veterinarian, and a quick Google search provides her credentials. The claim that any amount of lavender is dangerous for cats is exaggerated; even the linked article claims "a nibble or two of lavender might not hurt" a cat, but that large amounts, such as lavender essential oils, may be very toxic. Any responsible pet owner would regard a substance that could cause sickness to their pet as dangerous and something to be kept out of reach.