Super Mario Bros. 3 is an all-time classic, and particularly impressive when you consider the technology it relied upon. The 16-bit SEGA Genesis was right around the corner in Japan by the time the Famicom version launched in Japan, for example, yet Nintendo delivered what was a huge step up for its flagship series.
The overworld map added to its perceived scale, but the nature of memory management back in the day meant that Nintendo will have been using all sorts of coding tricks to squeeze all of the game's overworld locations, stages and minigames onto a humble cartridge. Now, with eager enthusiasts and ROMs at our disposal, these workarounds and coding are often deconstructed.
A recent example is from @BoundaryBreak, which shows that if you break the path boundaries in world one you can trigger the optional Mario Bros. stage; if you 'win' the stage your character switches to Luigi.
As many have pointed out in the Twitter thread the minigame isn't new, but its map location certainly is something different. It seems to be another example of how boundaries and coding in games like this can wrap around and lead to surprising skips and changes in progression.
It's never too late to discover new things in Super Mario Bros. 3, it seems.