The owner is right, the throttle is optional on a Greaser for legislation reasons.
Error 30 is often a general communication error.
I would try a throttle, from my limited experience it's frequently the last thing to fail and you could see the motor running.
I think they just tried to squeeze their hybrid into the legislation to make it street legal, hence the 250W European standard and the appellation E-bike.
With only a 250W motor, they should prepare themselves to pedal fiercely to move this thing with a front wind over a hill.
Power tools need a fast charging rate, sometimes in 1/2 hour. That is faster than 1C. Fast charging generate heat. Most E-bike chargers charge at 0.1C to 0.25C, 5 to 15 times slower than a power tool charger, a very soft charging rate.
IMO the best reason to touch the front brake lever and cut the motor is to pass a larger gear with legs power only. Powerful mid motors are strong enough to destroy the mechanics and shifter line switches are not reliable .
I suggest you to read or look at vids. about the chain line of e-bikes (Luna). I personally reduced my 9 speeds cassette to 6. I also touch my brake before shifting to stop the motor.
French dictionaries are full of borrowings from English, probably because it's practical or it comes from the usage, when it's a french word it's by snobbism ! Perception ? ;)