The word "artifact" was inadvertently omitted from Walking Atlas's type line. The card has received errata to correct this omission; it is an artifact creature.

Putting a land onto the battlefield as a result of Walking Atlas's ability isn't the same as playing a land. You may do put a land onto the battlefield even if it's an opponent's turn or you've played a land this turn. Similarly, putting a land onto the battlefield during your turn doesn't preclude you from playing a land later in that turn.