So this would mean: Only one subwoofer is producing the "real" sound and doing the hard work, while all the others are just producing the anti-sound? Wouldn't this be a massive downgrade compared to DLBC, where all subwoofers are producing the sound?
for the LFE (Low Frequency Effects) channel - yes
For all the other channels the workload is totally distributed and shared among any speaker (whether sub or not) that is capable within the required frequency range.
So if the AVR/Mixer-decoder shares the LFE load among any speakers that are full range - then with ART, they would handle all bass
But if (as is often the case) the subs only handle the LFE (and therefore do nothing with stereo for example, which has no LFE channel) - then yeah, no benefit.
Quite a few AVR's have a "double bass" setting, which feeds the LFE channel to the main L&R speakers as well.... but with ART something more sophisiticated will be needed - and it isn't about ART, but about the mixer/decoder, and how that then distributes signal to channels.