Yeah, that's about what the chain lasted on my D20 (Sram 1x11 with a KMC chain). I replaced it with the same as I needed a quick replacement, but next time I'll look around for options. 1,000miles for a well looked after chain isn't very good in my books. I've also developed a creak, and as of today a slight knock from my bottom bracket. Again having done only 1,800miles on my Gain, not impressed.
Lastly your mechanic is getting confused between e-bike drive types. With crank driven type drives he's right, extra power IS being transmitted by the chain and that's got to take its toll. On hub driven type bikes it isn't. In fact, surely the opposite should apply. Oughtn't a hub driven bike be easier on the chain?