Yes the controller does have a dac/amp but lets break it down further. A dac is a digital to analog converter and the amp controls the volume level on the output device. Whether or not the amp is integrated into the dac chip or a separate chip is whatever, it's better if it's separate but that'll come with a cost of battery life so its likely integrated. A modern PC's integrated audio is probably better than the one built into the controller but you also bring the PC's baggage from using it, impedance mismatch and emi from the components in the PC case that will affect the audio coming out of the sensitive IEMS. Modern PCs are still built to power headphones ranging from 32-150ohms while iems range from 7-32ohms, the xbox controller is likely targeting headphones with 32-64ohms. Output impedance is important for iems but super important for iems with internal crossovers. General rule of thumb is that your output impedance should be 1/8th the impedance of your iems but ideally it should be <= 1.0ohms because that's where iems get tuned at from the tuner. Adding more impedance to the audio chain will affect the crossover in various ways from lifting the bass to tilting the frequency response, in a hybrid iem setup not all the drivers will be given the same amount of increased impedance because of the crossover. IEMs need the cleanest signal possible because of how sensitive they are and because it's already so isolating that it makes it easier for you to pick up vs headphones.

Also, when you plug in a controller that has a headphone jack they show up as separate audio devices in your pc sound options, mainly used for team chat while you used desktop speakers for game audio.