I thought the SD card was only used for firmware updates, not for sending coins or signing messages. It could be I'm thinking of a similar wallet, but I've never heard of a HW wallet that works that way. That would be an enormous pain in the ass.
SD Card is just a way to transmit information, and technically anything that allows for a bidirectional transmission would suffice as a medium for transfer of signed and unsigned transactions. You can connect your computer directly to ColdCard through MicroUSB but SD card is just an alternative. IMO, it's just as about convenient as using a USB cable and the transfer is done in a jiffy.