The 2017 Honda Accord hybrid has a city EPA rating of 49 mpg which is mostly driven using its series hybrid mode. It also has a single fixed ratio parallel mode when driving steadily or with modest acceleration at speeds above 40-45 mph and that gets it an EPA highway rating of 47 mpg.Idk why you're comparing the serial hybrid to a regular non-hybrid ICE. That's not the question. It's serial vs parallel.
Serial hybrids intuitively seem like they should be very efficient if you can run a small engine at optimal rpm's. That's what I thought circa early 2010. But that benefit is overblown in reality. The Gen 1 Volt's weak 35 mpg_city -- even with premium fuel -- is a great example of this configuration's real-life limitations, as is the fact that no other automakers' conventional hybrids employ a serial configuration. They use a parallel configuration DESPITE its complexity precisely because it's more efficient.
The reason (explicitly stated in the article) for Nissan to choose a serial configuration for this vehicle is cost.
While a series hybrid is simpler and can skip the expense of a full transmission it has its own cost downsides. It requires 2 motors and inverter circuits that match the power of the gas engine whereas a car like the Hyundai Ioniq hybrid only needs one motor with about half the power of the gas engine but then still needs a separate transmission. A power-split system like a Prius or Malibu hybrid splits the difference by requiring 2 motors (one of them about half the power of the gas engine) but it uses a mechanically simpler transmission.