Just days before its release, Super Mario 3D All-Stars leaked onto the Internet in full. While a leak of the game is bad enough, an intrepid data miner reports that the game uses emulators to play the three retro Mario games within. In other words, the games are not ports for the Switch but are rather original ROM/ISO files (with some patches applied) played in emulation.
https://www.notebookcheck.net/Days-before-release-Super-Mario-3D-All-Stars-leaks-in-full-online-confirmed-to-use-emulation.494332.0.html