Shift coverage - barista vs ssv

An SSV I am friends with from another store had to get their shift covered. It was a coverage only shift (they were not at any point the 'shift on') and they had a barista cover it. Their SM, who is new to their store, told them that is not allowed. That only another SSV can cover, even if it's a standard coverage shift.

I checked the partner guide but all it seems to specify is that it's the partner's responsibility to "arrange for another partner to substitute... with management approval".

Does anyone know of any additional information on this and, if so, where it would be located?