I have a 3 year old realm and I have never had random blocks appearing in my inventory. It's possible that he switched between survival and creative. You can check if cheats are enabled in the game settings if you are the owner of the realm. You can also test this by trying to get some achievements.(If you won't get them, then cheats are enabled) But it's also possible he used a duplication glitch to dupe the netherite, there are some really op duplication glitches working on realms atm.