Hey there! 

Thanks for the question! It's a good one. =) 

Two things: 

One: The rule for that example is to highlight that a comma should NOT be placed between an adjective and the noun it modifies (yellow (adj) truck (noun)). 

Two: Great catch! There SHOULD definitely be a comma beteween big and yellow since it passes the "and" test (the big AND yellow truck). I must have left it out unwittingly, but I'm so glad you pointed that out! Thanks and great job catching the error! 

Stay curious and always keep learning! I look forward to more of your questions. 

Jin Teacher