This rule does not work with amable.  If you do an exercise on superlatives with a question on "amable" before you read the lesson on -co, -go, -ble, and -z endings you will make a mistake. Perhaps this lesson should should point out that -ble endings  are an exception.  

Of course I will always remember the rule now after spending some time trying to discover why  "amablísimos" was wrong.