The firmware can support SSH because TP-LINK added the support of Tether 2.0.
Tether 2.0 is an APP used to manage and configure routers; to guarantee the safety of the communication process; they use SSH to encrypt the data. So the SSH is available on the firmware.
What’s more, they have already considered the security issue, and have already blocked the related SSH tunnels and commands, which makes this SSH available only to the Tether APP. Other SSH clients cannot control the router via commands.