first you need to make a difference between digital out and analog out.
the X1 only has analog out and headphone out, no digital out. so it can't possibly bypass the DAP and be what we tend to call a "transport", sending only the PCM stream of digital data to an actual DAC+amp of your choice that will have to do all the leg work.
no the analog out like on the fiio X1 isn't the same as any other line out and doesn't send the exact same signal. you can make a caricature saying that an analog line out, is the signal output with the settings that offer the best signal possible for that given device(so pretty much the X1 with volume maxed out in this case, plus a change of impedance). so it's still very much the quality the DAC section inside the X1 is capable of with specs limited by the X1. it's very good signal, but not perfectly identical signal compared to another source or DAC.
now on the audible side of things, because the X1's line out outputs the best signal the X1 can create, the signal ends up being very good. and the perceived differences are usually small or inaudible. smaller than changing the amp section, and smaller still than changing the headphone. that's why using even an average source with a line out is usually a reasonable way to get good sound.
now if you were talking about actual digital output, and a DAP capable of behaving as a transport, then yes the signal sent should be 99.9% identical and entirely processed by the external DAC+amp. the 0.1% could be some jitter, noise coming from the transport and reaching a not shielded enough external DAC... small stuff that shouldn't have much if any audible impact as long as the external DAC is well made and the transport can use the best possible transfer protocol for the DAC(some will simply not work at all).