A MIDI basically only contains number: Pitch, length, volume, instrument ID (there are 128 of them), timing, etc. It does not directly contain any audio.
He did, in fact, use some of the same instruments as MIDI in a musical sense; he used things like a piano and a trumpet, which MIDI supports. However, MIDI relies on the OS or soundcard to supply the actual sound, to say "this is what piano sounds like" and create the audio on the fly. The remastered OST, on the other hand, uses custom sound files rather than simply what's included with Windows, instrument sounds that he purchased or downloaded that sound far better and allow much more control. I don't know specifically what virtual instruments he used, but as an example he may have used a piano instrument such as Alicia's Keys, which allows you to control how open or shut the lid to the piano is, damper AND soft pedals, where it sounds like it was recorded from, etc.
Furthermore, his remastered soundtrack no doubt had a ton of postprocessing done to it. There are quite a few effects that can be applied to a sound, track, or song, and the MIDI format supports very few of them (reverb and chorus, if I'm not mistaken). Again, you don't have as much control over even what effects are there, and how good they sound is entirely dependent on the MIDI patchset your system uses.
In other words, if he did use MIDIs, he likely would have used ones ripped straight from the game, and only as a starting point. You certainly wouldn't be able to get the nice, high quality sound that distinguishes his remastered OST with MIDI alone.
