No it's a good question. It would support playing the flac, but the power output on these players (like the m15s) is way higher than what you'd get from a $2-300 phone. Additionally, the 2.5 and 4.4mm ports are balanced outputs, which you're probably not going to get from a cheaper phone. The cheaper phone also likely won't support LDAC blue tooth.

It's that this device is more specialized towards rendering high quality audio and has the power output to drive demanding headphones that require a lot of juice to drive. The separation of instruments is more noticeable and you can crank up the volume extremely loud and there is no distortion to the audio, which is nice for parties.

The device I use daily also has a battery bypass function that enables "ultra high gain", so when I'm at home and want to drive my bookshelf speakers, I can turn the gain way up and feed the signal into another amp (using the fiio as a pre-amp to push more power to the amp). That way I'm not impacting the life of the battery in the audio player. The volume would be limited using a phone.

Edit: I fully acknowledged that the m15s is expensive, even on sale. I love listening to lossless music, travel regularly for work, and like spoiling myself with a high quality, portable setup. I use the player in my car, for get togethers, and while mowing the lawn. I've been very happy with the purchase and use the player daily. Enough power to drive my 660s and Shure 846 iems.