It is amazing that the loyal little penguin traveled so far to visit its friend, but that was just one example of true loyalty from a penguin. There are more dogs around humans, so there are more opportunities for them to show loyalty to people, and there are many examples of it. Perhaps if penguins became a common pet, they could steal the title of "man's best friend."
Dogs are and will always be considered man's best friend. This does not preclude penguins from also being known as man's best friend. Dogs are more prevalent as pets, and relatively easy to train, so obviously they have come to be more well-known as a loyal friend to man. So little is known about penguins now, but if they became prevalent household pets, they might also prove to be man's best friend.
Any animal can form an attachment with humans. Across their world, there have been many incidents where an man and beast have formed bonds stronger than family. From Christian the Lion who remembered the men who raised him before he was released into the wild to the little boy who bonded with a group of wild marmots in Austria, animals of all kinds have proved that the dog is not man's only best friend.
While a penguin that travels such a vast distance yearly to visit its rescuer is certainly a sweet story of friendship, you never hear of penguins rescuing people, so it seems like a pretty one-sided friendship to me. It's well documented that dogs have been known to save their masters in perilous situations. Heck, penguins aren't even our closest sea friend! Dolphins and whales have also been known to save people from death at sea. Penguins though, they always take, take, take.