Ah, the truth comes out! Someone just tapped the RCA outs and brought it to the front and called it done. This is wrong as the RCA out typically has high impedance in the order of 100 ohm we see here.
This is not true of Denon AVRs (unless something has changed) - that's a quote about a different product that MZKM wrongly assumed to be similar to the Denon.
They actually work as per the other descriptions above - they use the main power amplifier for the L+R channels fed through a 400-ohm series resistor.
This doesn't change the answer that any amp could damage headphones or speakers with too much volume.
Trying to protect your headphones or speakers by limiting your
amplifier power doesn't work. If the amplifier runs out of power when you turn the volume up, it clips, producing a distorted signal that could do more damage.
The only way to protect is to limit
gain - don't turn the volume up too much or have a device with limited volume.
I don't actually know whether or how much Denon AVRs choose to limit their volume when the headphones are plugged in. (They do know they're plugged in and reconfigure themselves, so they certainly could limit it). Without limiting, I believe that power amp + series resistor circuit would still be able to produce far more power than any headphone could handle, so I assume they do at least some software pre-amp gain limiting too.
The reason to go for a separate headphone amp would mainly be quality. The series resistor to the speaker output is something of a bodge. Might actually work better in frequency response for a low-impedance headphone like yours, but wouldn't compare to a dedicated headphone amp.
There would be some ease-of-use differences too, I guess. The headphone socket on an AVR is auto-sensing, and disables the main zone output, so you have to connect and disconnect the 1/4" plug to switch between headphone and speakers. With a headphone amp, you could permanently connect to the Zone 2 pre-outs, and just turn main zone (speakers) and zone 2 (headphones) on and off via the remotes, and use both simultaneously.
Oh, and you would be able to set a volume limit on Zone 2 in the AVR - exactly what you would want to make headphone damage impossible.