The fact I'm playing this at 20 FPS on Firefox with a RX 7600 shows how much web browsers have regressed in 10 years, not how much they've advanced.
Minecraft back in the day was a Java applet, so the game was running in a native JVM. This demo is running the game in a WebAssembly JVM, so performance is absolutely going to suffer. If the game were actually written for the web platform, it would perform significantly better.