There's various other torque sensing systems that measure chain tension or dropout deformation, too.

(And, IIRC, the power split systems that NexxtDrive and Revonte are developing can effectively calculate torque through their speed regulation methods - using Toyota terminology, they know how much power MG1 is pulling off of the rider to maintain a certain crank speed, they know how fast the rider is spinning the cranks, and they know the fixed torque split of rider input power to MG1 and to the output, allowing them to trivially calculate input torque (as long as, at least in the NexxtDrive version, the rider input isn't exceeding the output speed, if that happens rider torque bypasses the power split system and goes straight to the output - but then you know rider torque is high enough that you've maxed the system out).)