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I'd presume 'typo', but you typed it twice. What article did YOU read?

Regardless, I think the guy's mistaken. I think they're there to get their SSN cards. The SS letter is presumably the letter referenced here and the Passports, printed in Spanish, were likely issued by State in Mexico (and I bet they chartered the buses, too)

http://www.library.northwestern.edu/node/2894

Obtain a Job Offer Letter from your employer (NU office/department you are working in) that contains the following information: Employer identification number (EIN), name of employer, position title, start date, number of hours/week, supervisor's name and contact phone number and an original signature.
Using the job offer letter, request a Social Security Letter from the International Office through their document request system. Make sure to use the exact information from the Job Offer Letter to request the Social Security Letter to ensure that the two letters match. Bring the following documents to the Social Security Office
-Original Job Offer Letter written buy your employer
-Social Security Letter written by the International Office
-Original I-20 or DS-2019
-Passport
-I-94 card (the little white card in your passport)
-Social Security Application
Once your application for a Social Security Number has been approved by the Social Security Administration (SSA), the SSA will send a Social Security card to the address you provided on your application form. The usual processing time is two weeks. If additional security procedures are necessary, processing times may range from 4-8 week.

Why is the Student Visa important in this case?

The F1 visa category is reserved for academic students enrolled in colleges, universities, high schools, language training programs, and other academic institutions. The first step for a prospective student is being accepted for enrollment in an established school (University/College) which is SEVP certified.

Verified here

So I suppose the author should have taken a trip to Mahico, waded across the Rio Grande with his mother on his shoulders and turned themselves into ICE feigning 'no Habla' to get fast-tracked into the country. /s