Hybrids don't have alternators, for the very reason he gives, the petrol engine doesn't run all the time.
The 12v battery is recharged via the traction battery and can be recharged even when parked and the engine not running.
In fact Lexus recommend putting a little used car into the Ready state at regular intervals to allow the 12v battery to recharge from the traction battery. Should the traction battery charge drop low the petrol engine will fire up and start charging the traction battery, providing the transmission is in Park.
To answer your question, yes you could use a CTEK, connected to the 12v battery to keep it topped up. Left connected permanently it will maintain charge level automatically.