Although the website you reference is an official US gov't website,the information you quote does not tell the real story.  (Which is not surprising as many, many times the US gov't--like LE in general--has been caught lying to people.)

I, and others, are eye witnesses to the facts that at the San Ysidro crossing:
1.  The CBP has created special lines for people to re-enter the US without WHTI documents
2.  The CBP routinely allows people to re-enter the US without a passport (or other WHTI document)

That means one of two things:
1.  The information on the website you quote is incorrect in saying you must have a passport (or WHTI)
2.  The CBP is breaking the law by letting US citizens enter the US without a passport (or WHTI)

Which is it?

I'm saying the website you quoted is incorrect in saying a US citizen must have a passport (or other WHTI document) to re-enter the US by land from Tijuana.