Tax Strategies Book - Self Directed Investing Using 401k

[Anish Joseph]

I'm reading "Tax Strategies for the Savvy Real Estate Investor" now and the authors mention using a self directed 401k to invest in properties. All this makes sense except the example they use illustrates the point by saying the investor puts 40k into his 401k, how is this possible if the max contribution is 19,500? He gets a 13k tax break by doing this. 

Am I missing something?