You’re right, lectines are denatured and thus become harmless when heated once. Unlike they’ve claimed, lentils abd chickpeas aren’t harmless when raw, while peas are.

From the German Wikipedia:

Some vegetables contain lectins in their fresh state, which are toxic to humans or pets. These lectins are denatured by heating during boiling, frying or other forms of cooking and converted into a harmless chemical form. They retain this harmless form even after the food has cooled down again. Garden beans, chickpeas and soya beans should therefore only be consumed cooked.

However, compared to other beans, kidney beans contain an enormous amount of lectines:

The toxic compound phytohaemagglutinin, a lectin, is present in many common bean varieties but is especially concentrated in red kidney beans. White kidney beans contain about a third as many toxins as the red variety; broad beans (Vicia faba) contain 5 to 10% as much as red kidney beans.

Regarding the OC, using a slow cooker for preparing legumes is not recommended. Additionally, to reduce the content of lectines (and other unwanted substances), it’s recommended to soack dried beans prior cooking and discard the remaining water:

Phytohaemagglutinin can be inactivated by cooking beans for ten minutes at boiling point (100 °C, 212 °F). Insufficient cooking, such as in a slow cooker at 80 °C/ 176 °F, is insufficient to deactivate all toxins. To safely cook the beans, the U.S. Food and Drug Administration recommends boiling for 30 minutes to ensure they reach a sufficient temperature for long enough to destroy the toxin completely. For dry beans, the FDA also recommends an initial soak of at least 5 hours in water which should then be discarded. Outbreaks of poisoning have been associated with cooking kidney beans in slow cookers.

Wikipedia - Common Bean