It looks like an(other) attempt to split the battery charging functionality between "VOOC" phone and wallwart AC-DC converter, using "constant current" along the cable and therefore reducing incoming voltage.
So the answer to your first question is "it depends", other brand smartphones will reduce their power intake if they see voltage drop, so some phones might be charging, but always at lesser rate than their dedicated chargers.
To your second question, there is a misconception. Qualcomm QC doesn't use "constant voltage charging", the QC supplies higher voltage to provide more charging power to a phone across the SAME thin cable. The phone, in turn, CONVERTS this power into proper charging sequence, constant current first, then constant BATTERY voltage, all in accord with Li-Ion established technology.
Regarding your third question, "which one heats the phone more, voltage or current", the answer is "neither". Phones use DC-DC CONVERTERS based on switching technoloigy, so the heat is a function of net efficiency of those converters, all depends how well they are designed, with 70%, or with 96% efficiency. So the dissipation (and temperature increase of phone's body) might differ.
As it looks, the so-called "VOOC technology" just re-partitioned the standard charger by moving the CC-CV converter part of standard charger into the AC-DC plug, and leaving only battery wires directly to internal Li-Ion cell. Then yes, this schema will result in less overheating of the phone body, because the main (and power dissipating) functionality has moved into wall unit.
Finally, the article states that indeed their "micro-USB" end has six pins, which makes it a proprietary connector, which likely doesn't fit into any other normal micro-USB phone. Overall the whole VOOC is a pretty questionable proposition.
EDIT: Close inspection of the VOOC micro-connector indeed shows that it has 7 pins, not 6 as the sloppy article states:
It makes a bit more sense (power and ground are extended on outer sides), but clearly any normal cable won't fit into the OPTO phone.