I think 1 of the main reasons why this Lexus doesn't get much better fuel economy, and no better fuel economy than a regular Lexus is because the engine is not Atkinson cycle, but rather regular OTTO cycle.
In most hybrid cars, the engine is Atkinson cycle, where the intake valves stay open part way up the compression stroke to increase thermal efficiency. Thus it is more efficient not only in the city where the electric motors aid efficiency, but on steady state highway driving too.
But with this Lexus it is probably just plain OTTO cycle. If they were smart, they have camshaft behavior where it could change from OTTO to Atkinson and back on the fly, in millieseconds, so you can have more fuel economy and also the power when you get into it. But they don't.
Part time HCCI (Sparkless diesel cycle ignition) is even more efficient, with OTTO cycle when you want power.
Also if this car had a higher capacity battery pack with a more powerful electric motor, there'd be less need for engine displacement.