How do they power 150 LEDs from a TV USB Power Source?
...as Forward current of LEDs are 20mA.


Many LEDs have a maximum current specification around 20mA.  But turn down the usage (perhaps via PWM) or increase the resistance and they will consume less current (and not be as bright).

That said, I wouldn't trust that there's anything in this product to keep a long length of it from drawing more than 500mA.

The spec sheet says 4W/m, so 5 meters would be 20W.  A 5V supply would need 4A for that much power.   My guess is that if you plugged in the full length and tried to turn the brightness up, you'd either damage the USB port or the voltage would sag horribly.

It would be possible to add resistance in the strip to make sure 5m wouldn't draw too much.  But if they had done that, it wouldn't show the 4W/m power spec.  Instead this appears to just be a standard LED strip with an interesting USB control/power device.