Technically, it is higher resolution than their older games like Maniac Mansion and Zak McKracken (the first versions, anyway). Actually, wasn't Last Crusade the first LA adventure to use 320x200 graphics? I know they were all 320x200 technically, but MM and Zak's graphics (not the verbs on the bottom or text) had bigger pixel blocks and thus were a different resolution than what the game was rendered in.
And obviously, when they said 3D back then they meant depth of a picture, ability to walk behind and in front of objects, etc.
"Booyah! Look out, LeChuck! Here comes Guybrush Threepwood's glowing sword of hot monkey vengeance!"
-Guybrush Threepwood, Tales of Monkey Island