I had read a long time ago that a typical 3x9 arrangement has fewer "real" gears than 27. One reason is that you're not supposed to heavily use the gears that are the most opposed because it causes additional chain wear. Another is that some of the gears calculate to the same gear/inch.
Basically, there are 21 or 15 usable gears depending on how you want to calculate things. Then add to the equation that this is an electric bike and a single ring with 9 cassette gears seems reasonable.
This is really just tired old wisdom handed down from the days of wide inflexible chains. Modern 8-speed+ chains can cover the whole cassette with ease. Additional wear from the dreaded cross chaining is never going to be significant when the cogs already have profiled teeth. It's always funny to read the hand wringing from MTB riders about this when they don't even bat an eye at the horrible inefficiencies of 20psi fat tires.
https://forums.mtbr.com/drivetrain-shifters-derailleurs-cran... has a nice summary, including a picture from Sheldon Brown's website. https://www.sheldonbrown.com/home.html
Basically, there are 21 or 15 usable gears depending on how you want to calculate things. Then add to the equation that this is an electric bike and a single ring with 9 cassette gears seems reasonable.