They state 60 LEDs/m @ 18 watt/meter, thats 3.3 W per LED?
0.3 W per LED, they're assuming the standard 60 mA per diode for RGB 5050 SMDs.
By the looks of it if I wired in my 5 V, 20 A supply to both ends it would still set fire.
20 A is how much current the supply can provide, not how much it will actually give when hooked to the load. Those tiny ...