That's exactly right. The three pins of the output on a mobo ARGB header supply the +5 VDC and Ground connections for power to the LED's in the light strip, and a Control Line carrying addressed digital data packets. Along the strip, the LED's are arranged in Nodes. Each Node contains one LED of each colour needed, plus its own control chip that listens to the Control Line for a packet with its address. It does whatever that packet says with its own group of 3 LED's. So each Node can be a different colour at any one time. The power for the LEDs all comes from the two power lines, and the power required for the Control Line to signal the individual control chips is minimal. What a ARGB Hub does is get only that Control Line signal of data packets and relay that on down the light strip. Power for the +5 VDC and Ground lines for the LED's comes from the Hub's connection to the PSU, and none of that needs to be drawn from the mobo header. Thus, doing as you plan means feeding only the Control Line signal from the third output of the Splitter to the ARGB Hub. This is no strain on either the Splitter or the mobo header that feeds your entire circuit.
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