I'm not an engineer so someone can feel free to correct me if I misspeak here.
As Peng said, Impedance is basically resistance at a given frequency. So less resistance can be interpreted as easier flow of electrons. So correct in that interpretation I guess.
When dealing with speakers, amps and volume though, sensitivity is a rating of SPL (loudness) at 1W and 1 meter away. As you increase loudness, you require more power. Power is I^2 * R (current squared times resistance). So current might flow easier through that lower impedance, but more of it is required to achieve the same power (and thus SPL/loudness) at the lower resistance.
If you're trying to put out 100W into 8 Ohms, you require ~3.5A of current from the amplifier.
If you're trying to put out 100W into 4 Ohms, you require ~5A of current from the amplifier.
Most people will interpret that behavior as being "harder" on the amplifier. Which is where the disconnect from the Denon rep's statement is.
As far as sensitivity goes, it takes double the power to increase output (SPL) by 3db. So in order to get an 83db sensitivity speaker to be the same perceived volume as an 86db speaker, you automatically need twice as much power. Perceived "twice as loud" is an increase of 10db or 10 times the power, so you can see how power/current can quickly ramp up when sensitivity is considerably lower. SPL also drops off 6db (x4 power) every time you double distance (remember speakers are rated at 1m), so by the time you're 10 feet away, you're requiring x16 as much power to get that same SPL level from the speaker.