In the end , does it really matter ; if the OP is actually travelling on the flight tomorrow, the OP will be likely to be able to select seats today by checking in

I don't see that there is anything much more than speculation on why it may be unavailable to Sabre , which is iirc, where EF collects data

For EF issues, there is a thread in the appropriate forum that the EF staff monitor for Expert Flyer issues and where EF related issues would be best served; this is not the appropriate forum for expert flyer issues