The label of ‘sandbox mmo’ is already correct in describing what the game is. The issue is that people with ‘minecraft’ expectations (sandbox game, different from sandbox mmo) expect something different, or are unaware of the meaning in relation to MMOs.
“Sandbox MMO” is a single term with a separate meaning, different from “Sandbox” by itself (or ‘sandbox game’) which would be your creative mode Minecraft.
This was, actually, the first ‘type’ of MMO to be popular (Ultima Online, for example), and the term was created later (when things like WoW showed up) to differentiate ‘sandbox MMOs’ (no quests, players can do whatever, be whatever) from ‘themepark MMOs’ (game is on rails, people must follow quests, pick a class and be locked to it).
Full explanation in this old post,
Extra reading for defining the terms and their historical usage:
https://www.reddit.com/r/MMORPG/comments/2rztoi/what_is_the_distinction_between_theme_park_and/
TL;DR: It’s confusing to people that are more familiar with Minecraft and recent MMOs (most of which have been themeparks), I blame the gaming press for reusing the ‘sandbox’ term in such a different context.