Here's the footnote regarding the booking benefit
Preferred Seat Selection at Booking: When available, primary Cardmembers with a Southwest Rapid Rewards® Priority Credit Card or a Southwest Rapid Rewards® Performance Business Credit Card will be able to select a Preferred or a Standard seat at the time of booking, or up to 30 minutes prior to a flight’s scheduled local departure time. If no Preferred or Standard seat is available, Cardmembers will be assigned a seat in accordance with the fare rules of the ticket purchased. When available, Cardmembers will be able to select Preferred or Standard seats for up to 8 additional Passengers on the same reservation as the Cardmember, allowing for a total of 9 Preferred or Standard seats. If the Cardmember is removed from the reservation and there is no primary Cardmember or tier Rapid Rewards® Member with the same or greater seatings benefits, all seating selections for that reservation will be released and seats will be assigned in accordance with the fare rules of the ticket purchased. Benefits may not apply on itineraries booked with partner airlines. Cardmembers should allow up to 14 days for card status to be updated in their Rapid Rewards account to be eligible for this benefit. Authorized users of Rapid Rewards Credit Cards and employee cards of Rapid Rewards Business Credit Cards are not eligible for this benefit. Credit card account must be open and not in default when selecting a Preferred or Standard seat. If your Credit card account is closed or in default before or at the time of travel, the seat selection will be released. Chase is not responsible for the provision of or failure to provide the stated benefits and services.
It does not say that the cardholder must book from their account or use their own points to receive the benefit. As a cardholder, PP's husband has the right to select a seat regardless of whose account is logged in to book his ticket. That's why I'm saying it's an error: the system isn't properly recognizing that the seating benefit applies here because the programming logic isn't recognizing his cardholder status for whatever reason (likely that it's not logged into his account).
Personally, I would book the ticket in my account without selecting a seat, then log back in on his account and go in and pick the seat, since the text clearly indicates that he can book up to 30 minutes prior to departure. That is almost definitely faster than calling.
I hear you on being hesitant to call something an error versus a purposeful choice in the design. But, in this case, if the rule was that the cardholder had to be booking from his account or using his points, that for sure needs to be in the footnote and it isn't.