This isn't the 1-1 stage from Super Mario Bros. we remember.
Forge was one of the things that was delayed when Halo Infinite launched, and contributed to the disappointment many people felt regarding the support. Fortunately, it arrive late last year and since then, it has been continuously improved.
Today, it's possible to make pretty much everything with it, which includes Pokémon fighting, Tower Defence mini-games and... Super Mario Bros. The latter is literally a Super Mario Bros. stage made withing Halo Infinite, and while it certainly doesn't look as fun as Super Mario Bros. Wonder - it's still widely impressive to see what the fans can do with the Forge tools provided by 343 Industries.
Mint Blitz has made a video with a couple of these very alternative modes, all created with Forge by the community, including the Mario level. Check it out below.