What's actually inside a MIDI file, and how playback works
A MIDI file doesn't contain audio. It stores a list of note-on and note-off events, their timing, velocity and which instrument each track is nominally assigned to, plus tempo and time-signature markers in its header. This player reads that header to show duration, BPM and time signature, then lists every track with the instrument name it declares and how many notes it contains, all before you've pressed play at all.
Sound only exists once something renders those events into audio, and the instrument doing the rendering decides what it actually sounds like. This player renders every track through one sampled grand piano, regardless of what instrument the file names for that track. That makes a piano part sound accurate and a string or brass part sound like piano notes playing the same pitches and rhythm, which is enough to check what's there without needing a full multi-instrument synth.