Why does a resistor in my Neopixel data line work at all?
The resistor is there to limit the current into the input pin.  The input likely has a very high DC resistance (more than 1 megohm) so negligible current flows (on the order uA) and a negligible voltage drop is produced (on the order uV or mV).  The resistor is likely used to slow the slew rate of the connection (the input pin will have some capacitance, so adding a series resistor forms an RC circuit) to prevent overshoot and ringing which could cause issues with the communication.  It also prevents the I/O signal from trying to power the LED string through the ESD protection diodes by limiting the current to something that will not damage the internal diodes.