The Apex is available programmed for 2500W, and will undoubtedly be sold that way to most buyers. That's a ton of power and the 40 mile range is quoted for throttle-only. As the text says you'll get much more in pedal-assist, especially if you run it at PAS 1.

I presume the chain tensioner is intended to take up some of the shock between a big blast from the motor through the chain and hub.