I'm not sure why the incredulity from your end - I'm actually responding directly to the data that you provided and simply applied some math. EVs are about 4x as efficient as a gas car, so a 2% improvement to efficiency on a gas car will equate to an 8% improvement to efficiency on an EV.
If we don't want to take that as self-evident, let's walk through the data.
A gas car's overall efficiency is hypothetically about 25%, and no more than 28% (as per the Carnot Cycle). For example, the EPA itself states that only 16-25% of the energy from gas goes to the wheels. The same site cites 75-80% of battery-to-wheels efficiency for an electric vehicle. So right there we see 3-5x as efficient.
Now the breakdown here is important. For a gas car, rolling resistance is 4-7% of the entire energy available from the gasoline. So from your article - if you see a 2% overall fuel economy improvement, that means that 2% off of that 4-7%. Let's use worst case numbers, so 7-2 = about a 30% reduction to rolling resistance gives a 2% fuel economy boost to a gas car.
But for an electric vehicle, 25% of the overall energy is lost to rolling resistance. So that means that a 30% reduction there is 7.5% overall efficiency improvement. Hence my 6-8% number.
But in the case of the Bolt, it's potentially even higher. The EPA's numbers are mid 2010s and the Bolt is more efficient than the generalized numbers the EPA provides for electric vehicles.
The Bolt's motor is about 93.25 - 96.75% efficient (Fig 15) for almost all of its operating range. The Bolt uses an Infineon FS8000R07A23E3 single power inverter module to provide the DC-AC power to the motor, which has an IGBT3 style MOSFET system. While I can't find details for this particular module, studies show that IGBT3 inverters are about 96% efficient (page 79). What's exciting is there are new SiC based inverters that raise that up to 98% efficient, which would add considerable range to EVs. Lithium ion battery discharging is north of 99% efficient, but let's call it 99%. The Bolt has about 300W of idle power usage, let's call that 2%. So that's 6% + 4% + 1% + 2% ~= 13% drive system losses, vs the 18% that EPA is citing. So with less overall wasted, proportionally more is wasted on rolling resistance, and a 30% reduction there would provide even more savings.