so the meaning of the lesson

Isaiah, the lesson notes that this is the situation for 'most' adjectives. There are exceptions. There is a link in the lesson, for example, to the situation with words ending in a mute 'e'. From that lesson, you will also find links to other related lessons listed for specific situations :

 Adjectives ending with mute -e don't change in the feminine in French

The rule that nouns ending on -e are feminine applies to about 80% of these nouns. So you can't blindly rely on this. Examples are, e.g., le musée, le lycée, le livre, le judge, etc.