While I cannot say for certain that neurons are not the only thing responsible for these behaviors, I'm fairly confident in saying that we don't know nearly enough about neurons to assume that 7,400 equates to "pretty simple".
It's probably "pretty simple" compared to 340,000.
That said, I wonder whether some of the complexity was effectively pushed down into complicating behavior of the individual neurons. If there were strong evolutionary pressures pushing down the number of neurons (as seems likely, for so extreme a reduction) it'd be interesting to see what kinds of hacks were included to get it there.
