Actually, that's true. TREZOR T uses different software which is developed separately. Developers mentioned that it will be easier for them to implement new coins. Some of them will be backported to the older model.
Edit: Just checked their blog post describing the SD card slot to be utilized for data encryption. I hope it also can possibly be used to expand storage space in order to possibly install some form of wallet apps (like ledger nano S) to add additional altcoins.
I guess they won't have any in the stock for a long time. Expected official sale is somewhere in the Q2 2018. SD Card was said to be used for data encryption, storing labeling, Lightning Network channels information and some extra things needed for altcoins if there won't be enough space on the device.