This has apparently been floating around for a while now, but I'll be damned if anyone else wastes their time on this one. Please link to this in case the topic ever comes back up.

Tl;dr This video is not of legitimate, unmodified gameplay. I am willing to say it was recorded using Action Replay. The select dialog box is a red herring. I'll be posting the technical details below.

Edit: On retiming and checking my recreation, I was wrong about the message coming up early. I was counting video frames, and not thinking video frames are 30fps, so 11 ticks on the video is 22 frames in game. It doesn't change anything about the debug flag.

At 0x0053, a counter exists. This is Mike's forward, overworld movement counter. The counter ticks up from 0-15, and resets to 0 when Mike changes direction. This address is more than likely used to keep track of Mike's current position using mod division, which is why it counts up and not down. When the counter is not at 0, 15, or 16, the select press is blocked until it counts up to the nearest of those three. Once it reaches 15, or if it was already at 0 or 16, the input is accepted and a clock somewhere in memory (I haven't been able to find it yet, it may not count sequentially) starts, and 22 frames later the "You are in Chapter X" message appears. It cannot appear before this. If this timer's address was known, it would be trivial to reset it to any arbitrary number. The video linked above contains no cuts in the audio at all, and no visible cuts in the video. It does, however, start suspiciously outside of 3-1, after the countdown dialog. I suspect that before recording, two action replay codes were set, one of which changed that counter to 10 frames, one of which set the debug flag. They then walked on top of Miracola with collision disabled, pressed select, and buffered a right press with the text box. This gameplay is not possible within the game mechanics. I spent hours testing setups for this, and unfortunately, I have no choice but to conclude that this is nothing more than an elaborate hoax.

Video set to private within hours of this. I think that confirms it.