Dave Ramsey says RE should be 5% of portfolio

[Chris Wilkening]

I think most of BP would disagree with Ramsey's advice about real estate and debt, but there is one bullet here that states "Real estate should only be about 5% of your portfolio." I understand (but don't agree with) his opinions on debt, but this statement by Chris Hogan about 5% goes unexplained. 

Could someone help explain the reasoning here? Is there a reason, or is it just an arbitrary number?