You are somewhat right. The number looks better on DX90 for the most parts but when it comes to full output, you can see X3 and X5 can comfortable max out (and almost double the output) and still has THD under 1%. Basically it means DX90 should have the advantage as long as you don't push it outside of its comfort zone. The further you push it, the more X5 will catch up, and overtake when near / over 150mW is needed. But with that much power into 32ohm, DX50 should be just fine with most headphone out there.
iBasso stated it can do 2.8Vrms, but it is not that same as saying it is the maximum undistorted output. It is the 1% THD threshold that matters. Gain setting doesn't have much to do with it directly. What the measurement means is that you keep pushing the volume (and gain) higher and higher until you hit the 1% mark (or you max out on volume and gain, like X3 / X5 in this case). The 1% mark is chosen because it is believed that that is the point when distortion becomes audible to human ears. So basically it is a test to show the maximum power you can get into a certain load until you start to distort the sound audibly.