As far as rule #8,always assume a dealers undercard is a ten,is it really that far off? Say the dealer is showing a 5.So under any circumstance but everyone busting before him,he's going to draw a third card.As approx. 1/3 of the deck is 10s,it is fairly safe to assume that one of the dealers three cards will be a ten.
I think that message gets garbled into the " assume a dealers undercard is always a ten",but a very high percentage of a dealers three card hands will contain the aforementioned 10.