There is this note on the CSU impaction matrix for 2019-2020 admission cycle:
San Luis Obispo: Impacted for FTF and UDT in all undergraduate programs (and does not consider alternate majors).
Also on the SLO Website:
Selection to all Cal Poly majors is competitive and we have more qualified applicants than available new student spaces. Therefore, Cal Poly rarely considers alternate or ‘second choice’ majors. Choosing the major that best fits your educational goals is one of the most important decisions you can make as an applicant.
Maybe they have done away with the 2nd choice major for this application cycle. I would call admissions for confirmation.