hes saying that they both will flow 255lph, but the holley pump may flow that much at 50psi and the walbro will flow that much at say 70 psi.
Flow doesnt change based on point of measure. Pressure is resistance, Flow is amount moved, it is possible to flow the same amount at different pressures, as it is possible to have the same pressure at different flow rates.
Holley might flow 255lph at 50psi where the walbro flows 255lph at 70psi. or the walbro will flow 300lph at 50psi and the holley will flow 200 at 70psi.
witch means the walbro flows more at the same pressure, walbro just measures the flow rating at a higher resistance.
i see what your saying about bieng in the same size tube, but they dont test pumps in the same size tube because like you said you cant change flow without pressure in the same size tube. if they did every pump would flow the same amount at the same pressure.