Well, there is one thing to list the wrong spec while another to list an incomplete spec. What iBasso did is the later. Technically they are not wrong but the information they provided is so limited that it doesn't serve any good to the buyer. Unfortunately it is all too common for most manufacturer to leave in blank and way more convenience to let the buyer fill them in with assumption that most likely won't be true. Yes, you can say it is a way of marketing.
For raw driving power, I think X3 and X5 are both way above the rest and really pushing to the same level as a good dedicated portable amp. I won't say X3 is 'significantly' more power than X5, but I think it is safe to say both have way more than you need to get good result as long as you are not driving something like planner magnetic.
Both DAP are tested on high gain and max volume. I never get 3.1V on DX50 but I'll guess you will probably need a 75ohm load or higher. The problem with speculating power from spec is that some manufacturer uses uncommon load (or worst, no load at all) to get a good looking spec that doesn't hold up in the real world. For example, most DAP will measure beautifully without any load. Even a 150ohm load will look very well. But put a 16ohm load in, the result can be very poor, as lower impedance load is really going to stress the DAP and tell you what the hardware responses under everyday condition, given 10 ohm multi-driver CIEM isn't uncommon these days among audiophiles. In short, it is always better to take the measurement rather than hoping for the best.
Low gain max at 1.6Vrms, same 288mA current output into 47ohm load.
