Tyranids have on their stats a Armour rating like reinforced chitin (all 6) and natural armor: reinforced chitin. Does that mean their armor is 12?

No.

The trait "Natural Armour" indicates the creatures armour is "Natural Armour" which makes it vulnurable to certain attacks, such as hellfire rounds, which bypass natural armour.

The ONLY level of armour a character has is listed in its armour entry. FFG combines al of a characters stats together and lists them post-combined as well. If a Tyranid had armour 12 its armour entry would say, " Armour: Chitin (All 12)". Same goes for melee damage. A purestrain does 1d10+12 damage with rending claws NOT 1D10+12+8. The strength bonus is already added, just like all of the character's armour is already figured.