This Haki can be used as armor against strong attacks or devil powers.
This type of Haki also has the ability to bypass the powers of a Devil Fruit user, touching the "substantial body" beneath whatever protection the fruit provides, such as the case of body-altering Devil Fruits such as Logia or Paramecia users.
Venom Venom Fruit:
The Doku Doku no Mi is a Paramecia-type Devil Fruit that grants the user the ability to produce and control different types of poison [...] any person who touches the user will be poisoned.
So everything Magellan touches gets poisoned...
So if Luffy could have used haki, would that have prevent him from getting poisoned?