tout v toute. Abus starting with consonant or h aspire

Thanks for the exhaustive explanation of the subject, it’s a fabulous resource and definitely one for the notebook! One question I was left with refers to the tip:

 

This rule's main aim is to make sure the pronunciation is always [toot] in the feminine. Indeed, in front of a vowel and mute 'h', the last 't' of tout is pronounced because of the liaison, hence no need for the extra 'e'.

Is this saying that the extra e is optional, or is this a rule?

 

Many thanks 

Alison