No, not entirely, but you'd have needed to poke around a bit (pun intended - not sorry) to realise - if you stop it (I think just RUN STOP + RESTORE was sufficient at the right spots in the game, but if not, a freezer cartridge would make it trivial) and LIST, you get a huge BASIC listing, so a lot of it is BASIC. Here's a Youtube video going through some of it [1]. Especially at the end he lists a large chunk and you'll see there aren't a lot of SYS calls to machine code so it's not surprising people whould think so. In the comments to the Youtube video someone made this statement - I don't know whether or not it's accurate, but it seems plausible (you probably could do much of that in BASIC too, but especially the scrolling would likely be really slow and janky):

> It is mainly the text screens that are programmed in BASIC. The battles are all in machine code, including the sword battles, the ship sailing and the ship battles, the drawing of the terrain and the men walking on the land; basically anything that involved reading the joystick input and displaying sprites is all programmed in machine code and are called by SYS routines from BASIC. I spend many hours exploring the code and hacking the game by changing variables and memory locations.

[1] https://www.youtube.com/watch?v=WJOjSQ0yv8U