Build that walrus means succeeding at whatever task you are talking about at the time. When changed to "that's not building a walrus", it can mean someone did something bad.
Example one:
Boss: "Did you build that walrus?" (asking the employee if he finished the report he was suppose to be working on)
Employee: "Built!" (the employee is saying he finished the report)

Example Two:
News Headline: "Aliens announce attack on Earth"
World leaders: "That's not building a walrus"
by WalrusBuilder April 25, 2017
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